Pre Planning For Modifying A Fender Super Reverb Amp

[TomCarlos]

MEF Members....

As I continue to study up, test, noodle, etc, I have a project that I've wanted to dive into for some time now. Perhaps those that saw my recent post on the Peavey Vintage Amp know that I had an interest in that one- primarily the chassis, transformers, etc. I was hoping to use that and modify it to get a simple single channel amp. In lieu of that one, I am back to something that might be simpler from a design perspective. Building it will be another issue and thread.

My goal is to have a single channel tube amp, approx 40 - 50 watts, using two 6L6 tubes. I want a spring reverb but no need for a tremolo circuit (I have a Boss Tremolo pedal that works nicely). It would be nice to have a Mid range tone control. I'd like to have a compact cab with a single 12", 8 ohm speaker. I have an output transformer that will handle this power. I think I can get there by modifying a Fender Super Reverb (AB673) circuit, building a new turret board, and using something like the D-LAB electronics conversion of a Marshall Solid State combo to a Fender Princeton Tube amp as a model. That cab and chassis would be a good candidate for what I have in mind. There may be other donor chassis and cab candidates. The chassis works because of the pre-drill holes for the pots and other front panel controls. I'd need to punch holes for the tubes and accommodate the PT.

• I am posting the original Fender Super Reverb (AB763) below for reference.
• Assuming I would remove parts from the Normal channel and the amp would still work, I covered those on the modified schematic.
• I also covered the parts related to the Tremolo (that I will not use).
• I plan on swapping the Output Transformer in favor of an 8 ohm tap (that will match up to two 6L6 tubes).
• I also plan to swap the GZ34 rectifier tube in favor of four diodes (full wave bridge rectifier).

I am hoping that what remains in the modified circuit is correct and will work. I am concerned about connection points between the Normal and Vibrato channels and that I have not eliminated anything I would need to make this work; especially feeding the reverb circuit and returning that to the 7025 that will feed the Phase Inverter.

For the Power Transformer, it looks like the Classic Tone 40-18029 will work. That is one which is recommended as a replacement for the Fender 125P5D. The other option is the Hammond 290D2X. That transformer generates 710v (355 CT) as well.

I realize that going from a GZ34 to Diode Rectifier will raise the DC voltage. So I'm wondering if I should be looking at a PT with slightly less output - maybe 325 CT instead.

So for now, this is the plan - purely in "draft" mode. The nice thing about this is I can follow the roadmap already built and tested. I simply take the time to dive into the details and study the circuit as I go.

Comments are always welcomed- even if you see a big flaw in the concept!! Now would be a good time to get a reality check on this idea!

Tom

[Pedro Vecino]

I see it fine but if you want to copy exactly the vibrato channel you need to incorporate the 50K resistor (the value of the vibrato intensity potentiometer) over the junction of the 0.1 capacitor and the 220K resistor.
The other element that needs to emulate is the existence of the 220K resistor (at the end of the normal channel).

Also the 820R cathode resistor (marked as A) needs to be double value (1K5) since it was originally for two cathodes.

[pdf64]

You can’t use a full wave bridge rectifier with such a PT, as the HT will be 1.4 x 710, ~1kV
A two phase rectifier is the type to use for such an application, HT would be ~ 710 x 0.7

See http://www.valvewizard.co.uk/bridge.html

Why not use the PV PT?
EDIT OK the PV is a 4x6L6 amp, so maybe you don’t want the unnecessary weight?
I suggest the BF bassman / bandmaster replacement type PT, given they will be slightly lower winding voltage to better accommodate silicon rectification.
eg https://www.hammfg.com/part/290EX?referer=1063

[Daver]

Also, the junction of your 220K balancing resistors on the first filter cap stack needs to be connected to the junction of those filter caps.

[Chuck H]

Second what Pedro is saying. Often missed when people build stripped down versions of amps is the affect peripheral stages and mixing circuits have on the gain structures. It can be very important to keep the amp operating intuitively and even to keep symmetry in the wave form relative to gain structures. In my efforts to learn something about the design of guitar amps I've experimented with many 'variations' of known circuits and often found that the cut and paste method doesn't work as well as I'd hoped and the original design was more tweaked and engineered than is supposed.

And... In order for the bleed resistors in the power supply main filter totem to balance voltage across the 70uf caps you'll need to connect each one across each cap.

[TomCarlos]

Thank you for the replies!!

Pedro: I added the 50K resistor over the junction of the 0.1 capacitor and the 220K resistor - noted as (1) on my schematic. I changed the cathode resistor on A to 1.5K - noted as (2) on my schematic. To emulate is the existence of the 220K resistor at the end of the normal channel, I added the resistor, the .047uf cap, and the 100K that would normally feed the 7025 tube that was removed - noted as (3) on my schematic. I hope I did that correctly.

Daver and Chuck: Good catch on the Filter caps being tied to the center of the 200K resistors in the power supply. That change is noted as (4) on my schematic.

Pdf64: Thanks for the comments on the power transformer. If I could not find a suitable transformer, I could use half the secondary of a HV and the CT. As for the power supply, if I can find a chassis that will easily accommodate a Rectifier Tube, I will probably do that. But if pressed for space, I will go with the Full Wave Bridge Rectifier design.

For the Full Wave calculations, without knowing how much sag will be on the transformer, I am estimating at this point. But it does not need to be perfect science. The Hammond 290D2X is rated at 355v on the Red Leads (outer most of the secondary). The Hammond 290EX (the Bassman replacement) is rated at 330v.

Using the info from the Valve Wizard, here is what I see and using the Hammond 290D2X as the example (and copying text from the site):

“Typically, when the loading is light the transformer voltage will be 5 to 10% higher than the 'advertised' value.”

355v @ 5% higher = 373v
355v @ 10% higher = 390v

When the transformer is fully loaded to its 200mA rating the AC voltage will fall to the nominal value (355v).

The DC voltage will therefore fall to: 1.3 × 355Vrms = 461Vdc.

That gets me close enough!!

[Helmholtz]

I could use half the secondary of a HV and the CT.

That might exceed the winding's current rating. Unfortunately there's no way to parallel the 2 sections. Your amp will draw between 200 mADC and 250 mADC at full power. And with bridge rectification transformer AC current will be at least 60% higher than the DC value.

As for the power supply, if I can find a chassis that will easily accommodate a Rectifier Tube, I will probably do that.

That would be my recommendation. I tried SS rectifiers in BF SRs, '59 BMs, JTM45s and AC30s. Always preferred the GZ34.

[pdf64]

...If I could not find a suitable transformer, I could use half the secondary of a HV and the CT. As for the power supply, if I can find a chassis that will easily accommodate a Rectifier Tube, I will probably do that. But if pressed for space, I will go with the Full Wave Bridge Rectifier design.
!

You really need to grasp how fundamentally different the full wave bridge rectifier is to the two phase rectifier, and how a PT intended for a tube amp will be designed specifically for one or the other.
Because the rectifier type determines the ratio of not just Vac to Vdc, but also the ratio of Iac to Idc, and also whether the Iac rating is to be for continuous current flow (FWB) or a 50% duty cycle (Two Phase).
FWB; Vdc = 1.4 x Vac, Iac = 1.4 x Idc
Two Phase; Vdc = 0.7Vac, Iac = Idc

The same PT can't be used interchangeably with either rectifier type without massively affecting the HT supply capability.

[Helmholtz]

Acc. to standard literature Iac = (1.56.... 1.92) Idc for bridge and Iac = (1.11...1.35) Idc for two-phase rectification depending on conduction angle.

[TomCarlos]

Good point PDF... thank you for that info - Helmholtz too!

[TomCarlos]

Just watched an excellent video from the Uncle Doug channel on YouTube - Power Transformers: Basic Design and Function. In the video, he looks at a Fender Deluxe Reverb and calculates the requirements for a transformer. The assumption is that one would still be using the GZ34 rectifier tube. But he does a nice job on going through the schematic, adding up Filament current, Plate currents, etc.

A couple things that dawned on my today. (1) Of the transformers mentioned above, the Hammond 290D2X has a 5V winding. The heater winding is 6.3v CT @ 5A. The HV is 355-0-355 @ 300ma. The 290EX does not have the 5v winding. The other candidate would be the Classictone 40-18029. It has a 710CT @ 200ma, 6.3 CT @ 5A, and 5V @ 3A.

After doing some additional studying, I think I now get the point made by PDF and Helmholtz. Because of the continuous current flow in the Full Wave Bridge, the current requirement of the transformer would increase by a factor of 1.56 to 1.92 that of the Two Phase requirement. I will continue to look at that. It is a good thing you reminded me of this. Thanks again.

Back to the circuit, I hope my revised circuit comes closer to something that will work.

[Helmholtz]

Because of the continuous current flow in the Full Wave Bridge, the current requirement of the transformer would increase by a factor of 1.56 to 1.92 that of the Two Phase requirement.

I see you understood the principle. But the winding AC current requirement for bridge rectification is always 1.414 times (= square root of 2) higher than with two-phase.
The factor 1.56 to 1.92 is the ratio of transformer AC current to rectified DC current with bridge.

Often the amp's DC current demand is known or can be estimated from circuit parameters. OTOH, PT manufacturers typically only specify winding AC current. The factors above can be used to convert DC current to AC current. The conversion factor increases with reservoir capacitance. With tube rectification typically the lower value applies.

Example: 40W tube amp with GZ34 and 40µF reservoir cap. The amp will draw a HT current (DC) at full power of around 200mA. Multiplied by 1.11 (see post #9) gives max. AC current requirement as 222mA.
With bridge rectification AC current in the used part of the HT winding would be higher by a factor 1.4 for the same DC current, in other words Iac = 1.414*1.11*Idc = 1.57*Idc = 314mArms.

[TomCarlos]

I doubt this info is covered anywhere!! So I am adding to my notes and will be more careful in the future. Interestingly, it is dang hard to get specs on a Peavey Transformer. I guess you'd have to conduct lab tests to figure out the specs.

[Dave H]

I doubt this info is covered anywhere!!

This might come in handy. Hammond Design Guide For Rectifier Use.pdf

[pdf64]

This might come in handy. [ATTACH]57370[/ATTACH]

Have you had a good look through that page? There’s a fair bit of nonsense on it, but as it’s on Hammond’s site and there’s a few correct tidbits mixed in, it seems designed to lull you into a false sense of security