After selling my previous Mullard amps (and moving to the UK), it is now time to build a new pair.
Because I have a pair of Fostex FE207 - 95 dB/W(m) speakers, and want to keep the cost down, I decided to build a 20W version of the amp, but still want to apply the Claus Byrith mods to it. (Original CB schematics attached)
So how do I change this circuit to be a 20W version?
I am going to use Hammond 1620A output transformers.
Model 1620A
Audio Watts (RMS) 20
Primary Impedance 6,600 C.T.
Max. DC Per Side 158mA
Secondary Impedance 4/8/2016
Is it easy to do or a complete redesign?
Also, what voltage should those 0.22uF caps around the DC Balance pot (22k pot in middle of schematic) be, I lost my original notes. 😕
Thank you for any assistance.
Because I have a pair of Fostex FE207 - 95 dB/W(m) speakers, and want to keep the cost down, I decided to build a 20W version of the amp, but still want to apply the Claus Byrith mods to it. (Original CB schematics attached)
So how do I change this circuit to be a 20W version?
I am going to use Hammond 1620A output transformers.
Model 1620A
Audio Watts (RMS) 20
Primary Impedance 6,600 C.T.
Max. DC Per Side 158mA
Secondary Impedance 4/8/2016
Is it easy to do or a complete redesign?
Also, what voltage should those 0.22uF caps around the DC Balance pot (22k pot in middle of schematic) be, I lost my original notes. 😕
Thank you for any assistance.
Attachments
Extra power provides dynamic headroom. 
Just "recreate" what you had, prior to relocating to the UK.
Just "recreate" what you had, prior to relocating to the UK.
You need another power transformer.For 20W with Raa 6k6 it's a power voltage around 300volts with evt. 6L6 tubes. And then the problem starts, the phase inverter has not enough voltage to function well with those ECC83.Rather use a ECC82 and one change leads to another, resistors, preamp ...
The voltage on the capacitors in the bias circuit depends on the negative voltage from the transformer (after the rectifier), normaly 100volts will do.
Mona
The voltage on the capacitors in the bias circuit depends on the negative voltage from the transformer (after the rectifier), normaly 100volts will do.
Mona
Why not investigate some of the many European transformer makers, rather than pay the hidden duties and freight charges on an imported mediocre transformer? Sowter, and Monolith Magnetics are two I would consider, there are probably many others to consider that UK members could advise you about.
Why not just build a modern variant of the 5-20 with EL34s as I'm not sure that the difference in cost will be appreciable.
Why not just build a modern variant of the 5-20 with EL34s as I'm not sure that the difference in cost will be appreciable.
If your power is limited to the 20w output transformers why not just use the original individual cathode bias scheme for the EL34's? Makes things simpler and more flexible imho. That's what I did with my 5-20's and I used 25 watt output transformers.
Some things to think about then. It seems like the org. 5-20 will be the safest bet without any hassles. ;-)
The original has the front end wired as a pentode and they use 30db of global feedback. It still has way too much gain for today's sources. You will want to triode strap the EF86 and lower the amount of feedback to ~20db, you shouldn't need much more than ~20db to get good distortion numbers. I can open mine up and look to what I used for feedback values. You will need to dial in the correct feedback comp cap with the Hammonds, I used Edcor so my value cap won't help you.
EDIT: I see the CB schematic has the EF86 already wired as a triode so keep that arrangement.
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Actually Mullard specify a 7k OT. So while not ideal, it doesn't appear too far off. I also tend to agree with the other comment that it should be possible to get a more suitable OT locally.
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It depends on the screen taps used. For 43% they recommend 6.6k which he has. For 20% taps they recommend 7k. I used 8k w/ 20% taps and I was getting closer to 30 watts with cathode bias. Mullard states that @ 27watts out distortion is at .1%, I think they just conservatively rate the power on these amps at 20 watts to boast low distortion and good bandwidth.
I had modified Mullards as per the Claus Byrith design, which pushed them to 35W and had a REALLY good sound. I still have a complete set of valves (EF86, 12AX7 and EL34's) which I would like to reuse.
As mentioned here, the org Mullards have too much gain, which is why I want to use the CB design, but in a 20W version.
Just convert the power stage back to its cathode-biased form, and that will bring the power level down closer the original 20ish watts. Also keep the first preamp stage as the CB mod version, and keep the CB mod 150k and 390k resistor values between the PI and EL34s, so the power stage isn't driven quite so hard. That should get you to about where you're looking for.
Sent from my phone with Tapatalk. Please excuse any typpos.
Sent from my phone with Tapatalk. Please excuse any typpos.
To scale a 30W amp to a 20W amp, make the main B+ about 82% of before.
440V becomes 360V.
For such a small change, NO re-design should be needed.
But are you saving any money? Unless a just-right 360VDC PT is lower price than a 440VDC PT, it comes to the same cost. The "20W" OTs will work in a 30W re-make if you don't push big bass (and the FE-207 can only take so much without sounding strained).
> what voltage should those 0.22uF caps around the DC Balance pot
They hold G1 voltage of the power tubes. You 'could' adjust from "-60V" to zero (unwise!). Worst-case 60V, plus whatever tolerance for different parts (PTs often have a 50VAC bias winding which gives 70VDC) (wall-outlet may be 105V-130V or the equivalent in 230V lands). 50V caps will work until someone turns bias pot all the way. 100V is a much wiser choice.
440V becomes 360V.
For such a small change, NO re-design should be needed.
But are you saving any money? Unless a just-right 360VDC PT is lower price than a 440VDC PT, it comes to the same cost. The "20W" OTs will work in a 30W re-make if you don't push big bass (and the FE-207 can only take so much without sounding strained).
> what voltage should those 0.22uF caps around the DC Balance pot
They hold G1 voltage of the power tubes. You 'could' adjust from "-60V" to zero (unwise!). Worst-case 60V, plus whatever tolerance for different parts (PTs often have a 50VAC bias winding which gives 70VDC) (wall-outlet may be 105V-130V or the equivalent in 230V lands). 50V caps will work until someone turns bias pot all the way. 100V is a much wiser choice.
True, voltage scaling would also do the trick. That's also why I suggested using cathode bias, as that reduces the effective abide voltage.
Sent from my phone with Tapatalk. Please excuse any typpos.
Sent from my phone with Tapatalk. Please excuse any typpos.
Check with some of the more knowledgeable members, but couldn't the EL34 tubes just be connected in triode?
Use KT88's triode instead of EL34's UL. Its a bit less power (about 18W) but better sound. Don't use the EF86 as a pentode but triode connect it and change plate and cathode resistors. The power transformer will cope and drop the plate currents to 80MA per tube. Speaking from experience the original 5-20 design was upper middly.
The EF86 is already triode strapped in the CB mod variant of the 5-20 that the OP wants to build. What OT are you using with the KT88s?
Suggest checking with SOWTER AUDIO TRANSFORMERS. They have a range of output transformers including a high power 10K p-p for KT88 Wiiliamsons. I am using Australian trannys (A&R 5008) that are no longer available.
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