Help finding a drake 1202-133 PT for my Tube Rectified lead-spec 6ca7 frankenplexi

[JTM50]

Can anyone point me in the direction to get a vintage Drake spec 1202-133 PT? I have the 118, but am seeking the higher voltage xformer for more headroom. I have all marstran iron but don’t see it listed on their site.

If interested in my build here’s some specs it’s turning into quite the hybrid beast.

Split cathode
2.7k/.6
33k/500pf Lead tone stack
Big 4700pf bright cap
V2a

.0022 V1b coupling cap
Gz34

Next, I’m upping filtering as the amp is very soft and I want it to handle the gain better.

Going with 32uf preamp 50 pi/screen and 64 for mains (using thermistor to keep gz34 happy)
Lastly I’m throwing in some syvania 6ca7’s and hopefully the 1202-133 transformer.
Really proud of this build and would love any thoughts!

 

[playloud]

I don't recall seeing anyone offering the 1202-133 (perhaps Mercury, but their info is opaque and their iron is overpriced imo).

But you could fudge it with a 1202-55 repro. You'll get 450V B+ with a GZ34 rectifier - same as a JTM45. If it's a hybrid already, that seems like a fairly good option.

 

[JTM50]

I don't recall seeing anyone offering the 1202-133 (perhaps Mercury, but their info is opaque and their iron is overpriced imo).

But you could fudge it with a 1202-55 repro. You'll get 450V B+ with a GZ34 rectifier - same as a JTM45. If it's a hybrid already, that seems like a fairly good option.

Yeah I am considering that as well. Just considered about the current demand of the 6ca7s rather than kt66’s. Will that be an issue?

 

[JTM50]

Also would love any thoughts on the safety of 64uf mains with a rectifier. I’m doing all I can for protection using a thermistor to help sooth the current rush on startup as well as diodes if it ever does fail-Using a stout f32 sylvania gz34. So gut says it will be ok.

 

[william vogel]

Also would love any thoughts on the safety of 64uf mains with a rectifier. I’m doing all I can for protection using a thermistor to help sooth the current rush on startup as well as diodes if it ever does fail-Using a stout f32 sylvania gz34. So gut says it will be ok.

It’ll be fine but don’t use standby. Leave standby in play and turn amp on and off.

 

[playloud]

Yeah I am considering that as well. Just considered about the current demand of the 6ca7s rather than kt66’s. Will that be an issue?

No. I would increase the screen resistors to 1k.

 

[PelliX]

It’ll be fine but don’t use standby. Leave standby in play and turn amp on and off.

This ^.

I’m doing all I can for protection using a thermistor to help sooth the current rush on startup

It's a nice idea - I toyed with it, too - but realistically it doesn't actually work as well as one might think. There are some 'proper' solutions if you really want a standby switch, but they require a bit of re-wiring.

Now.... where are the guts?

 

[JTM50]

No. I would increase the screen resistors to 1k.

Does this just allow more current through? Would it be worth seeking out a 133 drake or would a 55 essentially feed the amp just as good?

 

[JTM50]

This ^.



It's a nice idea - I toyed with it, too - but realistically it doesn't actually work as well as one might think. There are some 'proper' solutions if you really want a standby switch, but they require a bit of re-wiring.

Now.... where are the guts?

What do these proper solutions consist of? Just curious on options.
And here’s the gut shots!
The things I’m still toying with is V1A and V2A, I have channels internal jumpered and want the normal channel to be like the high bright input of a jtm preamp. Right now it’s 1uf/1.2k but I’m thinking I need a larger cap like 25uf and possibly 1.5k resistor. Am inspired by bray amps as they do the parallel gain stages, and have great volume knob clean up.

 

[playloud]

Does this just allow more current through? Would it be worth seeking out a 133 drake or would a 55 essentially feed the amp just as good?

No - it burns off excessive current getting to the screens (and the voltage drop ensures Vg2 < Va). EL34s draw more screen current, hence the suggested increase.

I haven't actually played through a -133 (to my knowledge), but from what I've read, they have higher current capability. So expect the -55 to sag more (a matter of taste).

What do these proper solutions consist of? Just curious on options.

Merlin Blencowe has a suggestion near the bottom of this page: https://www.valvewizard.co.uk/standby.html

The things I’m still toying with is V1A and V2A, I have channels internal jumpered and want the normal channel to be like the high bright input of a jtm preamp. Right now it’s 1uf/1.2k but I’m thinking I need a larger cap like 25uf and possibly 1.5k resistor. Am inspired by bray amps as they do the parallel gain stages, and have great volume knob clean up.

You're not going to get the JTM sound with such a small cathode bypass cap (and the cathode bypass cap on V2A will also pose a problem).

Have you tried a more conventional 220-330uF/820R combo?

 

[JTM50]

No - it burns off excessive current getting to the screens (and the voltage drop ensures Vg2 < Va). EL34s draw more screen current, hence the suggested increase.

I haven't actually played through a -133 (to my knowledge), but from what I've read, they have higher current capability. So expect the -55 to sag more (a matter of taste).



Merlin Blencowe has a suggestion near the bottom of this page: https://www.valvewizard.co.uk/standby.html



You're not going to get the JTM sound with such a small cathode bypass cap (and the cathode bypass cap on V2A will also pose a problem).

Have you tried a more conventional 220-330uF/820R combo?

Yeah it was too dark with the mixer bypass cap bleeding the highs, but gonna go from 1uf to 25uf to split the difference a bit.

Also the higher voltage/more sag, might be a cool sound. That’s a good suggestion. Thanks so much!

 

[JTM50]

Yeah it was too dark with the mixer bypass cap bleeding the highs, but gonna go from 1uf to 25uf to split the difference a bit.

Also the higher voltage/more sag, might be a cool sound. That’s a good suggestion. Thanks so much!

And calling Chris Merren/Heyboer to see if they have that 133 PT.

 

[playloud]

A couple of other things I would probably try before switching the PT:
- Lowering dropping resistors to raise the voltages in the preamp/PI. That 8.2k dropping resistor would be a good place to start - maybe try a 4.7k there?
- Using a variac to increase the mains voltage (carefully!) Have you checked the heaters are dead on 6.3V? If they're below, you can safely increase the mains voltage until they are. If they're at or above, you can still safely increase the voltages ~10% (i.e. until the heaters are at 7V). This will give you a 10% increase in B+, so you'll get an idea if you like the change before plonking down $ for a new PT. Don't thrash the amp if you do this, and make sure you increase the screen resistors beforehand if using EL34s.

 

[PelliX]

What do these proper solutions consist of? Just curious on options.

I think the best in-a-nutshell-explanation to date is this:

The Valve Wizard

That said, it depends on what you're looking for in a standby switch. If you want a mute option, there are tons of places in the signal path where that could be implemented. If you're looking to swap cabs and such you could use the cathodes of the output valves, etc. Many ways to skin that cat, indeed.

The Ultimate Tone (TUT1) - Tube Amp Books by London Power

The Ultimate Tone is the book that kicked off the boutique amp market! 368 pgs; 395 schems. Kevin O'Connor's 1st volume on tube amps - an industry standard.

londonpower.com

Excellent reading if you want to snag a copy somewhere, too^.

 

[JTM50]

A couple of other things I would probably try before switching the PT:
- Lowering dropping resistors to raise the voltages in the preamp/PI. That 8.2k dropping resistor would be a good place to start - maybe try a 4.7k there?
- Using a variac to increase the mains voltage (carefully!) Have you checked the heaters are dead on 6.3V? If they're below, you can safely increase the mains voltage until they are. If they're at or above, you can still safely increase the voltages ~10% (i.e. until the heaters are at 7V). This will give you a 10% increase in B+, so you'll get an idea if you like the change before plonking down $ for a new PT. Don't thrash the amp if you do this, and make sure you increase the screen resistors beforehand if using EL34s.

yeah this seems like a good way to taste test. I’ll try dropping those resistors. Reached out to the iron kings to see if anyone has wound this xformer. Will keep updated on the other tweaks.

 

[JTM50]

yeah this seems like a good way to taste test. I’ll try dropping those resistors. Reached out to the iron kings to see if anyone has wound this xformer. Will keep updated on the other tweaks.

I think the best in-a-nutshell-explanation to date is this:

The Valve Wizard

That said, it depends on what you're looking for in a standby switch. If you want a mute option, there are tons of places in the signal path where that could be implemented. If you're looking to swap cabs and such you could ground the cathodes of the output valves, etc. Many ways to skin that cat, indeed.

The Ultimate Tone (TUT1) - Tube Amp Books by London Power

The Ultimate Tone is the book that kicked off the boutique amp market! 368 pgs; 395 schems. Kevin O'Connor's 1st volume on tube amps - an industry standard.

londonpower.com

Excellent reading if you want to snag a copy somewhere, too^.

The good ole’ TUT bible. I know there’s great stuff in there. One day I want to make sure my kit amp has optimal grounding but I know that’s a rabbit hole.

 

[playloud]

yeah this seems like a good way to taste test. I’ll try dropping those resistors. Reached out to the iron kings to see if anyone has wound this xformer. Will keep updated on the other tweaks.

I would also try Bryan Sours at Soursound. He probably doesn't have a -133, but I know he has reverse engineered quite a few vintage Marshall transformers and, if he has, the replica will be impeccable.

 

[JTM50]

talked to Chris Merren, and it seems his 1202-118 has a high voltage secondary that is 350VAC like a jtm45 PT. I have marstran right now and will have to check my taps. George at metro also says Heyboer has specs for a 1202-133.

 

[Kidsan]

Hi all, I just joined to say Drakes now owned by Powerstax, still make these transformers. The full part number DPTX-1202-133.
Made from the original spec, on the orginal machine by the same lady!

 

[2L man]

Can anyone point me in the direction to get a vintage Drake spec 1202-133 PT? I have the 118, but am seeking the higher voltage xformer for more headroom. I have all marstran iron but don’t see it listed on their site.

If interested in my build here’s some specs it’s turning into quite the hybrid beast.

Split cathode
2.7k/.6
33k/500pf Lead tone stack
Big 4700pf bright cap
V2a

.0022 V1b coupling cap
Gz34

Next, I’m upping filtering as the amp is very soft and I want it to handle the gain better.

Going with 32uf preamp 50 pi/screen and 64 for mains (using thermistor to keep gz34 happy)
Lastly I’m throwing in some syvania 6ca7’s and hopefully the 1202-133 transformer.
Really proud of this build and would love any thoughts!

Using tube rectifier which initially sag the High Voltage together with bigger filter capacitor(s) which first slow down the sag and then slows the "bloom"/recovery while increasing tube rectifier stress look strange to me. I know its been done but I use tube rectifier only when I want the HV sag and there also "correct" filter capacitance is good.

If clean headroom is needed then SS rectifier work better because there come only about half the HV sag what GZ34 bring and because of less sag bit less distortion.

If you mean NTC to restrict start up current it is one of the most unreliable component in electronics!

There is no need for NTC when amp has a Stby switch when resistor is installed to NO contact. Or two resistors for DP-switch NO contacts. This simple method make Stby switch operating perfectly!

Resistance can be select so that for example 1/3 of HV come to electrolytes which allow current flow thru tubes. I have use 33k for 12W amps. For "big bottle" 15k or 12k could be first but be prepared to change both direction alghough this "trickle" HV can be 100...200VDC.

Sometimes tubes produce noises when they are cold when amp is tutn On and "trickle" current warm them more naturally. Also cathodes do not "poison" which might happen when filament is On and anode current is Off.

If HV switch is installed to filtered HV the resistor over contact also stop or at least lessen a lot the pop sound which amp often produce when HV is switched back to Stby. Resistor kill better/faster the spark there come when contact open without resistor.