Traynor YBA-1 at high plate voltage

[Csquare4]

Hi folks,

As some of you know, the Traynor YBA-1 is a very close derivative of the classic Bassman/JTM-45 circuit. I recently picked up a spectacularly clean '67 copy that was all original with the exception of a rather "crappy" fully reversible master volume mod and unfortunately a burnt Power Transformer. This is the version that has the SS rectifier and choke and is exactly like the schematic that can be found here: http://www.lynx.bc.ca/~jc/661021_YBA1.gif

I spent quite a bit of time trying to locate a period correct PT, with no luck...these things were built like tanks and over engineered wrt the power circuits, hence, not many really bite the bullet. I finally ended up using the failed unit as a reference for locating a current production similar sized Hammond PT, which I found in the 278CX. Ironically, the form factor and mounting scheme was exactly alike, so all good there. Except - and this is a pretty big *except* - the secondary is wired for 800V center tapped. My calculations put the B+ at well over 500V. I have no idea what the specs are for the original, Hammond doesn't have a clue, as these were specially produced for Traynor in the mid-60's, and emails with Traynor came up short - apparently they don't keep records that long either. So off I go...

The original tube chart calls for 7027A power tubes, which were rated north of 500V on the plates and 450V on the screens. So I bought some of the CP JJ 7027A's, which are really just larger bottled 6L6GC's (I have found out), and installed the 278CX. While waiting on the PT to arrive I also did a bit of maintenance, 3-prong power w/death cap removal, cleaning the pots, replacing all the Electrolytics, checking the main coupling caps for DC leakage, installing a variable bias adjust pot/1-ohm 1% resistors off the cathodes, verifying and replacing key resistors that were in some cases way out of spec, etc. Typical stuff for an over 40yo amp that is all original. I also replaced the 4M Volume pots which is pretty standard mod for these YBA-1s.

I know this is long...so bear with me please...just laying down some background.

The PT arrived last week, I installed it, no problem. Well, ok, maybe a slight problem. If anyone is familiar with the *newer* 278CX, maybe you can offer some insight. The primary side is wired with White-Brown-Black leads for 115VAC and 125VAC connections. If you look at the spec sheet on the Hammond website, on this link at the bottom of the page:

Hammond Mfg. - "Classic" Power Transformer - (263-282 Series)

it shows that for 125VAC to use the White and Black. There is no gray, but mine is brown. No worries, I just assumed the brown and gray are more or less interchangeable. Ironically, on the label affixed to the top of the 278CX PT, it says:

115V: White - Brown
125V Tap: Brown - Black

I wondered about this - so after several days of trying to get in touch with someone at Hammond, I spoke to one of the engineers. He informed me that the spec sheet should be correct and for 125 mains, I should use the White-Black leads. Unfortunately, I didn't check before I installed it...I just assumed they knew.

So I wired it up and fired it up. Started testing some voltages, with no tubes I was in the high 500's on the B+ range, which was high but somewhat expected. However, my heaters seemed a little high...everything else checked out. So I put in the tubes. B+ dropped to 540V on the plates and 541V on the screens. Now that is HOT for a 50W amp. I checked the heaters and they are showing roughly 6.5V, even though it's in spec, its still seems kind of high. My wall voltage is right at 123VAC. So it made me wonder if I used the correct Primary voltage wiring scheme. I could disconnect the PT and manually check, but if anyone here has experience with this particular xfmr and knows for sure, please chime in. I just did such a nice job with my lead dressing I hate to rip it all out. Moral here, in the words of a great American: "Trust but Verify".

Ok - still hanging in there I see. Now to the jist. As I said the Plates and Screens are running right at 540Vs. I replaced the 470 1W screen resistors with a nice pair of Ohmite 1k 5Ws that I had just to be a little safer. I biased it down really cool due to the high voltages to see how it all sounds. I am at ~31 mA (@540 Vp) which corresponds to about 16-17W or just over 50% of max dissipation for a 6L6GC type tube. I watched the voltages for a while and everything was pretty stable.

So I plugged in my guitar, in this case is a '75 Guild Bluesbird with the original Guild humbuckers - which are pretty hot pickups. Speaker cab is a homebrew ported T-S 1x12 cab with EV12-L speaker. I turned the volume up to about 3 in channel 1, everything else was in the middle. Strummed an open E. HOLY SMOKES BATMAN! Talk about punch, clarity and extremely rich harmonics. Wow! Okay. So I started playing some chord progressions, tweaking the knobs...and shit! This thing has got some serious mojo. It is extremely clean with major bite, best I can liken it to is like an old BF Super but with some serious hair on the harmonics - which was really unexpected. Well, maybe not with these high plate voltages, but certainly not in my sphere of experience. I really didn't know what to expect with running these voltages in the late-JTM/early JMP type circuit with 7027 tubes. But damn! This is major sweet.

So now to the dilemma. Before I fired it up, after checking the voltages, I ordered some zeners to help tame some of the B+, but now...after playing this thing for about 2 hours last night - and with a tone that was fully capable of making me waste a pair of tubes just for the experience (!) - I have spent the better part of today on the web trying to find out about those high voltages. I have found all kinds of input, especially from the Ampeg crowd, which I suppose is use to playing with these types of plate/screen voltages, that may indicate that I should be ok if I keep the bias on the cool side. Obviously, the current production tubes leave a lot to be desired in "perceived" quality, but I am almost inclined to not change it up at this point. While there seems to be some warnings about running the plates - AND ESPECIALLY the screens - at this level, most of the folks that have commented across various boards that have Ampeg circuits seem to think this is acceptable if the overall dissipation is kept in check.

So I come to this board, as I have a high regard for the knowledge of a lot of the techs that visit here and pose the question:

- Should I worry about these voltages? Should I try to install the Zener dropping diodes to get below 500Vs? I am currently leaning to a resounding "no", but I guess I should get a few more insights from folks that may have some experience running at these levels.

So, as I await comments, I am now going to play - before the wife gets home!

Thanks guys!
Craig

 

[mickeydg5]

I do not know how much help will be given on this amp here. You may need to go somewhere discussing Traynor.

The YBA-1 calls for 7027A tubes. The JJ's are slightly less than the 7027A but better than the original 7027 spec.

Still I am not sure that your amp called for a 400-0-400 power transformer. I believe the YBA-1 MKII used the 400-0-400 with the 6CA7 tubes. I would think that the older model with the 7027A tubes would have a lower secondary voltage power transformer.

Hope this helps.

 

[Csquare4]

I do not know how much help will be given on this amp here. You may need to go somewhere discussing Traynor.

The YBA-1 calls for 7027A tubes. The JJ's are slightly less than the 7027A but better than the original 7027 spec.

Still I am not sure that your amp called for a 400-0-400 power transformer. I believe the YBA-1 MKII used the 400-0-400 with the 6CA7 tubes. I would think that the older model with the 7027A tubes would have a lower secondary voltage power transformer.

Hope this helps.

Thanks for commenting Mickey. I know this isn't a Marshall, but it is essentially the same circuit as the early Marshalls and I know a lot of folks like these as modding platforms...so I thought maybe people here might have played around with these. I wasn't sure if folks on here would be interested in commenting or not. I posed a few questions on the Traynor Amp Yahoo list a week or so ago and didn't get a whole lot of bites. Mostly everyone I think are used to the later units that had the smaller iron. I know the YBA-1A uses the larger transformer and sets 6CA7's/EL34's at about 550-560V on the Plates, but they have a different config on the screen node so that the screens are down much lower and the tubes are biased for about 80W or so. Those have electric fans on the head body or the chassis to keep to tubes happy. I am considering adding a fan, but not sure if I can do it without making it look kludgy. I am not about kludgy.

I was actually considering posting on M-E-F but Firefox has been screaming that they are hosting malicious content...which makes me leery because I had two PC's that I use frequently to access MEF come down with serious trojans in the past month.

On the 400-0-400 PT, the reason I bought this was primarily multifold:

1) Pure asthetics - it fit in the mounting holes and has identical form factor to the one that came out.
2) Current rating is very "beefy" at 535mA on the secondaries and 6A on the heaters.

I thought that I could easily shave 40-50V off of the B+ using zeners and maybe adjusting the dropping resistors if I needed to. But I also thought that this is the perfect platform for modding if I wanted to go that route...I had been thinking about maybe building a 6550 or KT-88 based amp, and this seems the perfect platform for that. Lots of room in this chassis and certainly big enough iron. So I still have that option. But now that I have played it, I am really digging what I have here.

So now I am just looking for insight on how I can expect these 7027A's to hold up under these kinds of conditions...if anybody has experience with them. If not, well, maybe I will just find out on my own.

 

[Lane Sparber]

Well, I'll throw my hat into this ring, for whatever it's worth to you.

First, to drop the voltage, you have a few options. I work for a lot of clients who have vintage Oranges and Laneys that put STUPID voltages on the plates and screens of their EL34s, so I've been down this particular road a lot.

You can use the Zeners, and that will shave SOME of the excess voltage off, but not enough. Even biased coldly, the screens are exceeding their maximum voltage ratings. This trusty data sheet specs just 400vdc as the MAX for these tubes' screens!

http://drtube.com/datasheets/7027a-rca1959.pdf

These high voltages might work today, and might work tomorrow, but sooner or later the screens will break down and the amp will fail...and more likely than not it will fail sooner. Also with the Zener diode option, if your bias supply voltage is tapped off of the B+ rail (and yours is), you'll have to adjust some resistor values in the bias supply to compensate for the missing voltage.

Another method you can use in conjunction with the Zener diodes is a Variac. It will dial your voltage down, but it's not a line isolation transformer so it's not ideal. Yet another option I've had my clients use is to buy a Japanese voltage converter box, which converts our 120vac wall voltage to 100vdc. These are cheap and effective. As long as your heater voltage doesn't drop below 5.75vac or so, you should be good.

Have you considered having a custom power transformer wound to your specs? I know Heyboer will do this for you, not sure about Hammond.

Finally, I'd like to humbly caution you that if you've got MORE than 500vdc on your heaters OR screens, you really should be using ceramic power tube sockets if you aren't already. Phenolic sockets can start to conduct at those voltages (especially when they heat up), and I've seen a LOT of plate to heater shorts caused exactly by this phenomenon. Check the link in my sig to some pictures I posted just a couple of weeks ago wherein I had to deal with this EXACT failure on two vintage Laneys with high B+ voltages. Really, if you're gonna run with these voltage levels, ceramic sockets would be the safest way to go.

Anyway, as I said, these are just my thoughts. Take 'em or leave 'em, and good luck with your new (old) amp!



-Lane

 

[mickeydg5]

Yes, it is similar to the Bassman & JTM45, but the power supply is a bit different.

Another voltage I am concerned about is at V2 pin3 to R14. What do you have?

 

[Lane Sparber]

Also, remember that a lot of Ampegs are/were dual-rail designs, which typically run the power tube screens at about HALF of the voltage of the plates - often using separate power transformer secondaries to achieve this. So before you act on what you read there, just make sure that you are, in fact, comparing apples to apples.

-Lane

 

[RickyLee]

Sounds like you have a similar issue that I have with an early 70's Mitchell amp that was designed to run two 6550's but it has aprox. 550VDC on it's power valve plates and the screens are only using the 1K 5W.




I put in a set of JJ KT77's and have no worries. Lane had mentioned to me about the good option of knocking down my B+ with the Zener as well, but I am just going to keep it with the KT77's or maybe pick up a set of JJ KT88's later on.

Regarding those JJ 7027A's, be careful running those at the high voltages. If you plan on staying with the 7027's, you can find some of the older real deal ones for a decent price as they should tolerate your high B+ much better. I picked up a few sets for decent prices for my Ampeg V4.

Also, I ran =C= 6L6's in my Ampeg V4 for quite some time with no issues at aprox. 540V on the plates and 537V screens. But yeah, that is pushing it. But I trusted going with the =C= before the JJ 7027 (relabeled JJ 6L6GC).

 

[Csquare4]

Well, I'll throw my hat into this ring, for whatever it's worth to you.

Lane, actually it is worth a lot, so I appreciate you taking the time to play. I got sucked into some Pearl Harbor activity on History Channel so I am a little behind on this thread.

You can use the Zeners, and that will shave SOME of the excess voltage off, but not enough. Even biased coldly, the screens are exceeding their maximum voltage ratings. This trusty data sheet specs just 400vdc as the MAX for these tubes' screens!

http://drtube.com/datasheets/7027a-rca1959.pdf

These high voltages might work today, and might work tomorrow, but sooner or later the screens will break down and the amp will fail...and more likely than not it will fail sooner.

I may be reading this wrong, also understanding that the JJ's are not old-stock RCA's, but it looks to me that on the bottom of page 2, under AB1 configuration, the following specs are stated:

Plate and Grid No.2 Max Voltages are both 600V.

I think the specs you are referring to are typical (or average), not maximum. Not trying to be argumentative, but that's the way I read it.

Also with the Zener diode option, if your bias supply voltage is tapped off of the B+ rail (and yours is), you'll have to adjust some resistor values in the bias supply to compensate for the missing voltage.

That's easy enough...I would expect that anyway. I replaced the 56k bias resistor in the bias circuit with a 22k and a 50k pot, so I have some leeway there, but if I need to replace the 22k with a different value I can manage that.

Yet another option I've had my clients use is to buy a Japanese voltage converter box, which converts our 120vac wall voltage to 100vdc. These are cheap and effective. As long as your heater voltage doesn't drop below 5.75vac or so, you should be good.

Now that might be really nice to have. I will seek one of those out for sure...never thought about one of those.

Have you considered having a custom power transformer wound to your specs? I know Heyboer will do this for you, not sure about Hammond.

I talked to Mercury and Hammond. Mercury will do it. Hammond won't. Haven't talked to Heyboer, but that is definitely a thought. Maybe a last resort if I can't get this sorted out.

Finally, I'd like to humbly caution you that if you've got MORE than 500vdc on your heaters OR screens, you really should be using ceramic power tube sockets if you aren't already. Phenolic sockets can start to conduct at those voltages (especially when they heat up), and I've seen a LOT of plate to heater shorts caused exactly by this phenomenon. Check the link in my sig to some pictures I posted just a couple of weeks ago wherein I had to deal with this EXACT failure on two vintage Laneys with high B+ voltages. Really, if you're gonna run with these voltage levels, ceramic sockets would be the safest way to go.

I checked out your photos on FB...yikes on those Laneys! May have been the original problem with this one now that I have seen those as the heaters were completely burned up from the pilot light to the PT. The light housing was toast too...so I knew it was a short in the Heaters, but I didn't think about tube arcing. Makes sense.

 

[Csquare4]

Yes, it is similar to the Bassman & JTM45, but the power supply is a bit different.

Another voltage I am concerned about is at V2 pin3 to R14. What do you have?

Hi Mickey -

When I was doing my voltage measurements I had written down 240V on V2 Pin3. That was the one parameter I was concerned about, but then when I looked at my notes for my JTM 45 and its at 205, so I figured I was around 15-20% - so I don't know whether I should be over concerned or not. I was trying to do a load line analysis on that triode earlier today, but couldn't really figure out if I was doing it right since there is no plate resistor and the output is from the cathode - I was gonna get Merlin's book off the shelf and dive in - then I got distracted.

 

[Csquare4]

Also, remember that a lot of Ampegs are/were dual-rail designs, which typically run the power tube screens at about HALF of the voltage of the plates - often using separate power transformer secondaries to achieve this. So before you act on what you read there, just make sure that you are, in fact, comparing apples to apples.

-Lane

The Ampeg I was looking at is the V4, which is one of those that doesn't have the separate PT for the screens. The schematic is here:
http://www.ampegv4.com/images/schematics/V4.jpg

If you look at the 7027a's in the Power section of that schematic, you will see that they are at 540V on the plates and 530V on the screens. That is precisely what set me off on this exploratory course to begin with.

Which, ironically leads us to Ricky Lee's comments:

Sounds like you have a similar issue that I have with an early 70's Mitchell amp that was designed to run two 6550's but it has aprox. 550VDC on it's power valve plates and the screens are only using the 1K 5W.




I put in a set of JJ KT77's and have no worries. Lane had mentioned to me about the good option of knocking down my B+ with the Zener as well, but I am just going to keep it with the KT77's or maybe pick up a set of JJ KT88's later on.

Which is along where I was thinking with the KT88's. Maybe I should try the KT77's as an alternative.

Regarding those JJ 7027A's, be careful running those at the high voltages. If you plan on staying with the 7027's, you can find some of the older real deal ones for a decent price as they should tolerate your high B+ much better. I picked up a few sets for decent prices for my Ampeg V4.

Also, I ran =C= 6L6's in my Ampeg V4 for quite some time with no issues at aprox. 540V on the plates and 537V screens. But yeah, that is pushing it. But I trusted going with the =C= before the JJ 7027 (relabeled JJ 6L6GC)

Yeah, I have had good luck with the =C= 6L6's in a few other amps, so I was thinking I might order another set - especially with the deal Tube Depot has on them right now. You can get a matched quad for $120. Almost a steal these days. Don't know about the old stock, I have the original tubes that came out of the amp, GE 7027's, which I was almost tempted to plug in and see if they had any life, but then when I look at the burnt out PT sitting on my desk, I think better of it. I haven't priced the NOS copies yet, maybe I will spend some time looking into those as well.

 

[mickeydg5]

About 200V is ok for the 12ax7A. 240V is way high. All of your B+ voltages are going to be in the neighborhood of 20% too high. C20 & C21 are in jeopardy.

You should have bought a 291DX. That 278CX is too much. Did you ask for that or did they recommend it?

 

[Csquare4]

Hmmm, ok. Like I said, I was curious about that one. So I can probably play with the slope resistor value a little and see if I can drop it down - if I intend to keep these high voltages, not sure yet. Maybe I can not screw up my tone stack too much by playing with that one, which is currently a 47k. Also, the 100k cathode resistor may be able to change too, I haven't really looked into this yet.

Once I get the Zener's I will see what I can do to tame them a bit then play with these and try to bring that voltage back onto the ranch.

On the 291DX, the form factor is a lay-down, I need an upright. But yes, spec wise, it probably would have been a more logical choice. No one recommended either one. Neither Yorkville/Traynor or Hammond had any clue what the specs were and my appeal to the Traynor board didn't really provide any real insight either. Like I mentioned earlier, of the few folks that responded, they were typically talking about the wrong amp or the wrong model. Traynor started making some pretty drastic changes just after this and one of the cut backs was the cheaper PT's that came in the later models.

 

[mickeydg5]

Just to reiterate
All of your B+ voltages are going to be in the neighborhood of 20% too high. C20 & C21 are in jeopardy.

 

[Csquare4]

Just to reiterate
All of your B+ voltages are going to be in the neighborhood of 20% too high. C20 & C21 are in jeopardy.

Yes they are - I see that now! Good catch on those caps, I didn't think to check that they were under spec'd at these voltages. Okay, now I am convinced I need to do something. Thanks for pointing that out.

 

[mickeydg5]

A 100V converter will drop the heater voltage too much, not good for tubes. It may hit 5.4V and that is for a good tube.
You better check into a custom transformer. That may cost you $500 plus. I do not hink you need custom, just the right one.

A true 7027A can handle those voltages, but you are taxing a JJ tube.
You need to worry about the rest of the amp too.

 

[Csquare4]

A 100V converter will drop the heater voltage too much, not good for tubes. It may hit 5.4V and that is for a good tube.
You better check into a custom transformer. That may cost you $500 plus. I do not hink you need custom, just the right one.

A true 7027A can handle those voltages, but you are taxing a JJ tube.
You need to worry about the rest of the amp too.

Right on. But DAMN it sounded good, lol.

 

[mickeydg5]

It will still sound good.

If you are handy, L-brackets could be made for mounting the 291DX. The leads on that one are 9 inch long.

 

[RickyLee]

You can upgrade your filter caps. That's what I did to two of my old Traynors.

 

[Lane Sparber]

A 100V converter will drop the heater voltage too much, not good for tubes. It may hit 5.4V and that is for a good tube.
You better check into a custom transformer. That may cost you $500 plus. I do not hink you need custom, just the right one.

A true 7027A can handle those voltages, but you are taxing a JJ tube.
You need to worry about the rest of the amp too.

Yeah, if he has a Variac he can test this heater voltage at 100v to make sure, and the Zener/Variac combo could get him safely where he needs to be as well. For the record, I agree with you in that I believe that a new custom made or alternate tranny is the way to go here. There's just too much damned voltage with this tranny!

My central point was that he does have a couple of options to consider if he feels he MUST use this transformer. It goes to show -and I mean absolutely NO disrespect to anyone in saying this- that we shouldn't always just drop something in there just because it physically fits. Consideration of the component in question's electronic properties and how it will affect the circuit always come first.

-Lane

 

[Csquare4]

Yeah, if he has a Variac he can test this heater voltage at 100v to make sure, and the Zener/Variac combo could get him safely where he needs to be as well. For the record, I agree with you in that I believe that a new custom made or alternate tranny is the way to go here. There's just too much damned voltage with this tranny!

Not giving up yet. I have a variac available but I need to pick it up from a friend. I will play with that and the zeners to see where I can get to. As I mentioned, this was an amp I purchased to mod if needed, so I went overboard on the PT. I can always replace that if I can't get to where I need to be...then maybe I will do that KT88 build from scratch.

My central point was that he does have a couple of options to consider if he feels he MUST use this transformer. It goes to show -and I mean absolutely NO disrespect to anyone in saying this- that we shouldn't always just drop something in there just because it physically fits. Consideration of the component in question's electronic properties and how it will affect the circuit always come first.

-Lane

No offense at all here...trust me, I searched high and low for specs for this particular unit. I knew they were over-engineered wrt PT's and I wanted to try to keep from mutilating the chassis. I went with the lowest power that would fit the form-factor and expected to have to play with the voltages to make it work. Not really surprising. What was surprising was how good it sounded when I tried it out.

Regardless, this is fun experimenting. Keeps me out of my wife's hair. I will update the thread once I figure out my final course of action. I really do appreciate you guys taking the time to comment.