Supro 1624T ptp layout with switchable "1690TCoronado" V2B triode

[Blues]

Hello, for anybody interested, here’s my custom take of the Supro 1624T with (switchable) additional preamp triode, just like in the Supro 1690T “Coronado”, the fabled Led Zeppelin I amp.

I followed the well-known schematic from Eric Kettenburg (Dirty Girl Amps). Layout was entirely conceived and drawed by myself.
Given that I built it hoping to get the Led Zeppelin I guitar tone (which was due to a modified Supro 1690T "Coronado"), I decided to add a switch to the basic circuit in order to activate the V2B triode, present in the 1690T "Coronado" but left unused on the 1624T. While wiring it, I followed the exact "Coronado" schematic.

For chassis, I used the 040 - PX18/PI18 from Tube-Town. You can find the re-designed panels in the following posts on a pdf file which is ready to be uploaded and printed from Tube-Town custom laser shop.

Transformers are Hammond 372DX as Power Transformer, and Classic Tone 40-18090 as Output Transformer.

About the OT, Eric Kettenburg explained on another forum that “the original 1624T output transformer is practically identical to the OT used on the Tweed Deluxe, just a touch fatter as it had @ 1 or two extra e-i leaves and the coil probably had a few more windings as some of them measure up as high as @ 9.5K to 3.2 ohm, but they were all over the place - some as low as 7.7 to 7.8K… a high quality tweed deluxe OT makes a fine sub at 8K to 8 ohm and works well with 6973 or likewise 6CZ5/6CM6 (as long as your screen feed is on pin 1 for both output tubes). There are also a few manufacturers who make 'bigger' tweed deluxe OT, closer to the size of a tweed bandmaster/pro. Classictone makes a "deluxe" deluxe OT they claim @ 20W handling (clean) and that one has 4/8/16 taps on it which would give you even more speaker variation. That's a great OT for these amps”.

Heyboer APD-8090H (sold by Amp Parts Direct) or Hammond 1750EP should be perfect for Output Transformer, then.

I used Vishay 2W metal film resistors and Vishay/Beyschlag ceramic capacitors, given that the latter show a kind of microfonic character which seems to be crucial for that ratty Supro sound.

Speaker is Jensen P12Q, the same speaker used in the Jimmy Page “Sundragon” Led Zeppelin I amp, which is refered to be 95% sounding as the old Rola speaker mounted on the original one. Cabinet was custom built with pine wood and Vox grey tolex.

Output tubes are RCA 6CZ5 military spec’d for the West Germany government, whose sigil is impressed in white on the glass for QC approval.
I was lucky enough to buy six tested pairs, matched and completely brand new from a very reputed european seller, many years ago, for a literal crust of bread - way more than enough for covering my lifetime use.

The amp works perfectly but is not really that interesting in its basic form, has very low volume and sounds pretty bland. Things change radically while switching in the additional triode, which gives a serious volume and gain increase, reaching the oh-so-much-craved Led Zeppelin I tone with a Telecaster with proper ’59 style bridge pickup.

Tremolo works, too, but it is very subtle and uninspiring and lefts large room for improving and modifications.

I hope to link some properly recorded demos of the actual amp in a couple of months.

[Blues]

Here are some pictures of the cabinet and the control panel.

[Blues]

Here are some pictures of the chassis and the inner circuit.

[Blues]

Here is the file with the printable panels according to Tube-Town laser shop requirements.
Please notice that the switch hole is not drawed and has to be drilled on your own.

[bmccowan]

Very nice! You took your time on the board and it shows.
I'm a bit of a Valco nut myself so have a few questions.
I assume the Eric K schematic you refer to is the 1690T schematic?
Eric now makes gorgeous black powder guns - he is quite the craftsman.
His old posts on the Gear Page are under EFK.
I am not surprised that you find the tone of the 1624 to be a bit underwhelming. I have found the same unless they are cranked up near max. I find the 6V6 and 6L6 amps to be more to my liking.
But I am surprised that you describe the standard config as "very low volume." With good power tubes like those you used, I find the amps have good volume. You may want to search for a problem.
And I and I'm sure others would love to see your schematic associated with that layout - care to share?
Again - nice build.

[Willabe]

And I and I'm sure others would love to see your schematic associated with that layout - care to share?

Yes, please post the schematic.

[Blues]

Very nice! You took your time on the board and it shows.
I'm a bit of a Valco nut myself so have a few questions.
I assume the Eric K schematic you refer to is the 1690T schematic?
Eric now makes gorgeous black powder guns - he is quite the craftsman.
His old posts on the Gear Page are under EFK.
I am not surprised that you find the tone of the 1624 to be a bit underwhelming. I have found the same unless they are cranked up near max. I find the 6V6 and 6L6 amps to be more to my liking.
But I am surprised that you describe the standard config as "very low volume." With good power tubes like those you used, I find the amps have good volume. You may want to search for a problem.
And I and I'm sure others would love to see your schematic associated with that layout - care to share?
Again - nice build.

Hello and thanks for your kind appreciation.
The low volume in stock form is probably associated to any kind of issue (wrong resistors value?), any help would be absolutely welcome. Anyway the amp sounds very good with the additional triode, as per 1690T "Coronado" configuration.
The schematic I made reference to is of course the 1624T by Eric, too, which is pretty much the same of the 19690T "Coronado", except for some values and the already mentioned V2B additional triode.
You can find both schematics attached here.

[bmccowan]

Thanks - but did you draw up a schematic of your final circuit?

[Platefire]

Don't forget the other half of the LZ 1 tone equation is a classic Tele

[bmccowan]

And don't forget the other 90% which was J.P.

[Blues]

Thanks - but did you draw up a schematic of your final circuit?

No, but actually could be pretty appropriate - I have to do the most difficult thing ever, in order to do it: find the time

Don't forget the other half of the LZ 1 tone equation is a classic Tele

I have a proper one Partscaster with American Professional neck (maple fretboard) and Warmoth body (One piece extra light Swamp Ash), White blonde nitrocellulose lacquering with black pickguard, Glendale bridge plate with top loader holes and grooved steel saddles, Don Mare Zep0tone Studio bridge pickup, Di Marzio twang king in the neck with bypassed tone control still missing Jimmy Page hands, tho'

[bmccowan]

Hey Blues,
An amp I built a while back is pretty similar to your build. It's Valco inspired 2 channel with 1 channel using two additional gain stages and a tone stack and one channel skipping all that and going directly to the PI. Preamp tubes are 5751s and 12av7 - power tubes are RCA 6cz5s. The hot channel gets that Z1 tone around 2pm on the vol pot. The normal channel is pretty lifeless unless dimed. I used a Mojotone chassis and faceplates for a Vox based amp and Hammond organ iron. I also like the amp with 6cm6 power tubes which use the same pinout and are closer to 6v6s.
I think you could easily use the schematic as a template to draw out yours. Do you use ExpressSCH or Jschem?
Attached is a schematic in sch and pdf.

[Blues]

Hey Blues,
An amp I built a while back is pretty similar to your build. It's Valco inspired 2 channel with 1 channel using two additional gain stages and a tone stack and one channel skipping all that and going directly to the PI. Preamp tubes are 5751s and 12av7 - power tubes are RCA 6cz5s. The hot channel gets that Z1 tone around 2pm on the vol pot. The normal channel is pretty lifeless unless dimed. I used a Mojotone chassis and faceplates for a Vox based amp and Hammond organ iron. I also like the amp with 6cm6 power tubes which use the same pinout and are closer to 6v6s.
I think you could easily use the schematic as a template to draw out yours. Do you use ExpressSCH or Jschem?
Attached is a schematic in sch and pdf.

Hi and thanks for the input! I have to draw the complete schematic, indeed - I never used any software, so I can't tell between ExpressSCH or Jschem, I hope to check them as soon as I can! So many things to do, so little time... 

[SpeedTrip]

Hoping you do get chance to post a schematic, I've tried to work out how you did it from the photos. Im toying with the idea of building a version with 4 12AX7 tubes. Using one for the tremolo as in Steve/Suckeys layout, one for the phase invertor and then one each for the two circuit , each one having the second stage switchable as you've done. Probably 6v6 output tubes and feed the tremolo into the phase invertor circuit as I think the modern Supro versions have done. Early days yet though, hence interested in how you've designed it.

[Blues]

Hello, sorry for the prolonged silence.
Here you can find the schematic I based my layout on - the switchable stage is reported on the lower side, and is assumed to be added with the switch between the red separators you can find in the main schematic.
I'm taking advantage of this new reply to agree on the point that the stock version I built (read: with the additional stage switched away) is unnaturally tame and quiet for being normal - I guess there is some point where the signal bleeds to ground, or something like this.
And tremolo works pretty badly, too - almost no oscillation.