My first plexi! (‘69 Super Bass, opportunities for schadenfreude within...)

[playloud]

Well, I finally got my first plexi! Some of you may recall being pestered with rookie questions when it first appeared on ebay (thanks again for your collective help with that), then again when it reappeared on Reverb. I was able to negotiate a reasonable price with the seller, and (in lieu of shipping) have him drop it off at a highly-regarded repair shop in his local area.

The shop gave it the once-over, installed new filter caps, changed the tap on the mains transformer, and (fastidiously) crated it, before sending it across the Pacific Ocean to me in New Zealand.

When it arrived, I noticed that the shop had set the transformer tap to 220V. The mains voltage is nominally 230V, but it's not uncommon for it to reach 240V (I measured 241V from the power socket during testing at one point).

After conferring with the tech however (for what it's worth, a well-known expert), I was convinced that the amp would be able to tolerate this, and got to spend about 30 minutes enjoying its magical tone. I have owned a Germino Club 40 previously, so I knew I liked the bass circuit, but this is my first vintage plexi and was definitely on another level. I started with the strat to warm it up, then switched over to the LP and started jumping channels. (I had asked the shop to revert the split cathode on V1 to shared, but that got lost in the communication somehow - either way, I thought I'd take advantage before I converted it back myself.)

This was interrupted when, with both channels at around 8, and the guitar volume around 6, the HT fuse blew!

Holding my breath that was just a tube (and not, say, the OT!), I replaced the fuse and gingerly fired the amp back up. I couldn't see any unusual flashes or color in the tubes, and I checked for loose sockets or shorted pins. When I switched off the amp however (I skipped standby, to discharge the filter caps before testing other things), the mains fuse blew! Disheartened, I decided to sleep on it before touching the amp again.

I consulted the tech back in the US, and he confirmed that everything tested well in his shop, including the tubes - and they tested it at volume. He also said that as long as I had a reactive load of the correct impedance attached (which I did, albeit a Boss TAE rather than a speaker cab), OT damage was highly unlikely.

The next day, I checked the bias (I couldn't see redplating when I was running it, but I'm definitely no expert). The pin 8 readings all looked fine (25-30mV), but the plate voltage was 600V on the one I was brave enough to check! I played the amp for a little while (lower volume this time), but after about 20 mins, there was a "saggy" drop in volume and a mild burning smell, and I decided it was time to take it a local tech...

It's with him now, and I should hear the verdict tomorrow. Hopefully it's just a matter of moving the tap to 240V and replacing some tubes (maybe a NOS quad - if I can find one?) Cross your fingers for me...

Here are some more shots of the amp. Couple of things I noticed were the Hunts caps (I assumed these were modern replacements from the original photos, but look to be original) and the fact that the only change to the circuit seems to be that split cathode.

'69 Super Bass

• playloud
• Mar 6, 2021

'69 SB Internal

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• Mar 6, 2021

'69 Super Bass Turret Board

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• Mar 6, 2021

'69 SB Back

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• Mar 6, 2021

'69 SB Tubes

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• Mar 6, 2021

 

[neikeel]

Glad that you got it
I would always use the 240v tap at our mains voltages
600v is simply too mych for most modern (and a lot of older) El34s on the screens
Laneys run that by design and trash modern EL34s (although JJs do hold up) I only use the Siemens/RFT in the Supergroup era Laneys and as a routine I swap the 470k screen resistors to 1k5 (a token I know but I've not had failure since.
Whatever is going on it looks like a very decent plexi SB, personally I would put V1 back to stock as shared cathode with a 100uF or 250uF WIMA 6v cathode cap and 820R resistor removing the extraneous bits.YMMV

 

[playloud]

Thanks for the vote of confidence (and your help when I was considering it)! Yes, I think I'll give the shared cathode a go.

I actually missed an opportunity to pick up a matched NOS quad of Sylvania 6CA7s a couple of weeks back (another guy got them before I had the chance), and now I'm really regretting it. I do have some old Mullards for the preamp valves, but nothing for the power section. Will start looking, as I suspect the plate voltage will remain high even with a different mains setting. Are Siemens/RFT particularly tough?

 

[neikeel]

No you are getting 10% higher voltage than you should
A normal PT with correct voltages is 480-490v on the plates even if you have a hot PT you should still be under 500v.

 

[Ned B]

I imagine the heater voltage exceeding 6.3V would be a problem. If it is still running high on the 240 tap maybe adding a VARIAC to fine tune the voltage would be a good idea.

 

[Derrick111]

The green Hunts signal caps are original to that amp and can be found for a short time period around 1968. I would think your amp is a '68 or very early '69 as they are not usually seen by '69 and I've mostly seen them in 50 watters, though that doesn't mean a thing except that may have been what I have only come across to this point over the years. In fact, I still have a 50w from '68 with those green Hunts in it.

The Siemens/RFT tubes are not robust like JJ, but they sound very good. The caveat is that they plates can only handle a maximum of something like 475v (but look that up to confirm the actual value as I am doing this from memory).

Yes, the mistake here was running the amp at this voltage setting. You can always test if you are not sure that your amp is running in the proper range by testing your heater filament voltage. If it is not within 10% of 6.3v, you adjust your power supply accordingly. For us in the USA, since sometimes the power transformer taps are not properly designed to handle our 120v, the amps often run a bit hotter and they are already often at the edge of operating perimeters when things are proper. To check if that is the case, I simply check the filament voltage. You don't even need to though, the 220v tap is your problem. Switch the power transformer in the amp to the 240v tap and test the filament voltage and I bet you will see a big difference as well as in your B+. Let us know what it reads aster you do!

Cheers

 

[Matthews Guitars]

The amp's voltage tap is set at 220 volts but your line voltage is 240 volts. That's a bit over 9 percent more voltage on the output side of the mains transformer than the amp is designed to work with. Every voltage in the amp is thus running too high.

There may be other problems but START with setting the voltage tap to 240!

 

[shakti]

As neikeel says, that amp deserves to be put back to stock on V1. That cathode cap is ridiculously oversized and overspeced, not a good thing in that position. Try to find a WIMA or Radiospares 250uF 6V cap as he says, they pop up on Ebay. I think the RS cap showed up frequently around that time, a little earlier it was almost always the WIMA.

 

[boola1]

I use a voltage regulator which shows the line voltage at all times. My voltage can get up to close to 250V. It makes me wince thinking about a 100watter on the 220 tap at that voltage!

6.3V is the way to go!

 

[Seanxk]

You never know but just check with our experts, which colour wire? has been connected as it may not actually be the 220V

 

[playloud]

You never know but just check with our experts, which colour wire? has been connected as it may not actually be the 220V

That has crossed my mind. The amp is still with the tech, so I am hoping he will confirm that (and switch to 240). The tech in the US was very confident that it was the 220.

 

[playloud]

I'm thinking voltage regulation is the way to go in future. Unfortunately, there seems to be a gap in the local market between the cheap, too-good-to-be-true stuff and the high-end Furmans (the cheapest I can find that actually regulates is a P1400-ARE which, at >USD2k, is way overkill).

Perhaps a variac with a mains meter on the socket (so I can keep an eye on it) is a fine alternative?

As neikeel says, that amp deserves to be put back to stock on V1. That cathode cap is ridiculously oversized and overspeced, not a good thing in that position. Try to find a WIMA or Radiospares 250uF 6V cap as he says, they pop up on Ebay. I think the RS cap showed up frequently around that time, a little earlier it was almost always the WIMA.

What's the value on that cap? I thought it was massively oversized. Will definitely return to stock.

 

[boola1]

I'm thinking voltage regulation is the way to go in future. Unfortunately, there seems to be a gap in the local market between the cheap, too-good-to-be-true stuff and the high-end Furmans (the cheapest I can find that actually regulates is a P1400-ARE which, at >USD2k, is way overkill).

They do appear occasionally used on ebay or reverb. I lucked out on a brand new for £700. Yes, still quite expensive but it does a great job. It can deliver 230 or 240 so you can get really close to 6.3V. My amps sound consistently good now, no more good and bad days. If you have a few vintage amps, I do really recommend one.

 

[yladrd61]

No you are getting 10% higher voltage than you should
A normal PT with correct voltages is 480-490v on the plates even if you have a hot PT you should still be under 500v.

My 1970 Super Lead B+ is a little over 500 with input voltage regulated a 117 VAC with a Variac and Line Voltage Regulator with EL34s and measures right at 6.3 VAC on the heaters, it is in the high 490s with 6550s. It is hard to find a Line Voltage regulator for 240v at a reasonable price you would be best to follow the advice to use the correct tap on the PT and use a Variac to fine tune the input voltage.

 

[Im247frogs]

rob robinette has a very cool little bucking transformer scheme and diagram on his website- you can drop you wall voltages in steps at the flick of a switch. I've been thinking about making one just to check it out. Couldn't hurt and it'd probably be safer than running any of my amps thru my 40 dollar chinese variac.

 

[Derrick111]

You never know but just check with our experts, which colour wire? has been connected as it may not actually be the 220V

That's why you test the filament voltage... easy way for most people to tell if things are right or not.

My 1970 Super Lead B+ is a little over 500 with input voltage regulated a 117 VAC with a Variac and Line Voltage Regulator with EL34s and measures right at 6.3 VAC on the heaters, it is in the high 490s with 6550s.

That's typical for that era Marshall... nothing wrong with your line voltage, so a "regulator" isn't going to do anything. If you lower your line voltage, your filament will decrease as well. You just have to be mindful to use robust tubes like JJs, make sure they are matched, and make sure they are biased properly. This is critical in British guitar amps having higher B+. The OPs issue is different... he had 600V+ due to having the voltage set wrong.

 

[yladrd61]

My wall voltage usually measures well over 120, I use a Variac to keep it at 117 as that is where the heaters are right at spec. I don't particularly care for the sound of JJs, I usually run Mullard Blackburn XF2s if I am running EL34s.

 

[Derrick111]

Mullards can handle that too, but most cannot afford hundreds of $$ for a set of output tubes and most are not NOS by this point. That is why I mention JJs.

 

[yladrd61]

Mullards can handle that too, but most cannot afford hundreds of $$ for a set of output tubes and most are not NOS by this point. That is why I mention JJs.

The early 70s Metal Panel Marshalls are definitely hard on power tubes. Shuguang and Psvane also make some very rugged EL34s at a great price, they also make some very nice high end audiophile power tubes. I am thinking of trying some of the Psvane KT88s in there.

 

[shakti]

What's the value on that cap? I thought it was massively oversized. Will definitely return to stock.

Can’t really see what value it is, but it has the physical size of caps intended for higher voltage applications. If it is around 250 uF as it should be, then I would guess it has a voltage rating of 50-100 V based on the size of it. It will work just fine, but it looks out of place and most likely a NOS cap of the proper rating will sound better too. It shapes the frequency spectrum of the whole amp, being the cathode cap for the very first (input) stage, so it’s more important than you’d think.