Twin Reverb 135w UL Mods Question

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dorrisant
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Twin Reverb 135w UL Mods Question

Post by dorrisant »

There is a local kid (still in high school) that just bought this amp... I think $300 bucks. He is the lead singer/guitar player in a pretty darn good band. He is also really interested in knowing how things tick so I am going to help him by teaching him how to bias his own amp while we get it ready to gig with.

I already changed the screens and grids on the power tubes. He wants this one to be Blackfaced, so I was going to change the PI values - 330ks to 1Ms and the 47ks to 82k & 100k... in the yellow box.

I want to be able to adjust the bias too. I was thinking of changing the 3.3k resistor to a 10k pot... probably 10-turn with a lock. I want to leave the balance pot but I'm wondering if I can't change the 47k and 68k to 220ks (typical AB763 values), just off of the Output Tubes Matching pot... in the red circle. Is there something about the UL that I am missing? I read something about removing the 12k just to the left of the PI. Gonna try that and see. I still have a hard time trying to understand the Master volume switching arrangement, so any explanations are welcome. How much can I change to BF values? What about NFB?

Can I delete the Master Volume and the associated switch and replace it with a Lar-Mar type? I have put a few in a Twins that lead to smiles...

I am just thinking that the owner and I can both learn something here... and it will be easier for him to re-sell if it's not his cup of tea.

Feel free to flog me, apparently I need to learn something.
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martin manning
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Re: Twin Reverb 135w UL Mods Question

Post by martin manning »

The preamp is almost blackface spec now, it's just the MV, the PI resistors, and the UL output section that aren't. The MV looks like it drops in a treble bleed (eliminating that 12k disables that) and attenuates the reverb. I think a PPI MV might be a better

The tough part about a conversion is that the B+ at the reservoir is over 500V, and there isn't a screen node filter or a choke. The PI node voltage is about the same as AB763, thanks to the 2k7 10W. I think I would be tempted to keep the PT and add a choke followed by a filter cap for the screen node after the 2k7. If you can determine the OT primary impedance that would be good to know for adjusting the screen voltage if necessary.

For the adjustable bias, yes I think replace that 33k with a 50k pot or better a pot and a resistor to idiot-proof it. The 68k/47k vs. 2x 220k will have an effect similar to reducing the PI output couplers from 0.1 to 0.02-0.03, so I'd expect a bit more bottom with that.
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Re: Twin Reverb 135w UL Mods Question

Post by shane »

Leave the UL as it is and lose the NFB, if you want it warmer run one of the pairs in triode mode too. Works well.
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Post by Stevem »

Ditch the master volume and the crappy boost circuit then use the switch on master volumes back to brake open the right hand wire on the rear of the intensity pot to stop the signal draw / drain of the Trem circuit.

Yank out the #2 input jack on the Vib channel and in stall a 3 position rotory switch to change its first gain stage bypass cap to .068, 5 uf and 22 uf . This will let you keep the big PI coupling caps and drop out the unneeded low end as you crank up the amp, then do the rest as above, but I think you should leave the feedback hooked up if you do the Trem mod I posted here!

Some other nice tweaks will be to change out the recto's for fast recovery types and change out the 470 2 watt resistors on the screen for 5 watters as they have roasted for years sitting on top of the output tubes, the same for the 1.5k resistors , but they can stay 1/2 watt.
Also paralleling another 220 uf filter with the ones in the amp now makes for a super punchy amp and really helps since there is no choke in these amps, you just have to silicone glue then in around the PT area and insulate them off the chassis with fish paper .
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tictac
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Re: Twin Reverb 135w UL Mods Question

Post by tictac »

Some players actually like the PI in the Silverface amps like this. I Blackfaced the PI for Jim Campalongo's (Nora Jones) SF Pro Reverb and he had me change it back stating he liked the tone better. He thought it had a kind of a compression; what I really think he meant is it was cleaner at loud volumes....

In any event the plate load values seem to be more in line with what you'd expect to see with a 12AT7 tube....

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Re: Twin Reverb 135w UL Mods Question

Post by pdf64 »

To make the bias adjustable, first off I suggest to increase the bias supply voltage, by reducing the 1k2 to 100 ohms and the 2k2 down to 1k or 470 ohms.
As the original bias voltage is not sufficient for some modern tubes.
The '3.3k' resistor off the balance pot is really 33k.
I suggest to replace it with a 20k or 25k pot in series with a 15k or 22k resistor.
Those 135 watt TRs can sound great, with a bit of sympathetic maintenance; I doubt changing the circuit to AB763 would improve it.
They can sound too bassy as the bass control taper is ~30% rather than ~10% of previous versions; if it's a problem, try a CTS audio taper 250k, as their standard audio taper is 10%.
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martin manning
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Re: Twin Reverb 135w UL Mods Question

Post by martin manning »

Changing the bass pot taper won't reduce the bass, it only changes the control setting for a given bass level, no?
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Re: Twin Reverb 135w UL Mods Question

Post by bluesfendermanblues »

martin manning wrote:Changing the bass pot taper won't reduce the bass, it only changes the control setting for a given bass level, no?
True, but IME like on dumble amp, it makes it so much easier to dial in the bass.
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Re: Twin Reverb 135w UL Mods Question

Post by dorrisant »

martin manning wrote:I think I would be tempted to keep the PT and add a choke followed by a filter cap for the screen node after the 2k7. If you can determine the OT primary impedance that would be good to know for adjusting the screen voltage if necessary.
I'm interested in this...

Primary is 2300Ω with a 4Ω load...

10vac into primary and 0.42vac across the secondary

1/0.42 = 2.38... X 10 = 23.8...

24 to 1

24 X 24 = 576

576 X 4 = 2.3k
Last edited by dorrisant on Fri Oct 20, 2017 3:53 am, edited 1 time in total.
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martin manning
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Re: Twin Reverb 135w UL Mods Question

Post by martin manning »

That'll work. The DCR of the choke will drop a few volts, but you might try adding some series resistance in front of it to get the screen down 25-30V below the plate. A choke and a cap (22 uF, 500V) are pretty cheap and easy compared to a replacing the PT.

Note above I should have said add the choke and cap to ground between the 2k7 10W and the standby switch. This will add a node to run the screens in the conventional way, and then the 2k7 will drop the voltages down to Blackface levels for the PI and preamp.
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Re: Twin Reverb 135w UL Mods Question

Post by martin manning »

Actually, you might want to use two 47uF 350V caps in series for the new screen node to cover transients.
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Re: Twin Reverb 135w UL Mods Question

Post by dorrisant »

martin manning wrote:That'll work. The DCR of the choke will drop a few volts, but you might try adding some series resistance in front of it to get the screen down 25-30V below the plate. A choke and a cap (22 uF, 500V) are pretty cheap and easy compared to a replacing the PT.

Note above I should have said add the choke and cap to ground between the 2k7 10W and the standby switch. This will add a node to run the screens in the conventional way, and then the 2k7 will drop the voltages down to Blackface levels for the PI and preamp.
I totally get these two points...
martin manning wrote:Actually, you might want to use two 47uF 350V caps in series for the new screen node to cover transients.
How does this help cover the transients?

I'm sorry if it sounds stupid but how does the screen voltage relate to the OT impedance?

I realize you are human and make errors... I have read your posts long enough to realize that this happens from time to time. You must realize, along with most here, that you are one of the resident gurus and I totally believe you are right, like 99.999% of the time, (not doubting you here). I just need to understand more of the basis for the theory... I was very good at understanding an algebraic formulas in school. Once it made
logical sense, I would never forget it, and could apply it in normal day life more readily. Not trying to kiss your @$$, just giving due respect... and maybe incentive to continue to answer my stupid questions.

I feel that sometimes someone has to be the one to ask those questions in an intelligent way for the sake of all the ones that are too hesitant to do so.
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Re: Twin Reverb 135w UL Mods Question

Post by Roe »

I recommend the tad audio series 20uf 550v cap made in germany for the screens
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martin manning
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Re: Twin Reverb 135w UL Mods Question

Post by martin manning »

dorrisant wrote:
martin manning wrote:Actually, you might want to use two 47uF 350V caps in series for the new screen node to cover transients.
How does this help cover the transients?
When you switch the power on there is likely to be some ringing, and the peaks could be higher than the steady-state DC. Here that's 500V, so the screen node filter should be rated higher than that. A compact and inexpensive solution is a couple of 300 or 350V radials in series, with balancing resistors (like a Dumble PS), which could be tucked inside the chassis somewhere near the power tubes on a terminal strip.
dorrisant wrote:...how does the screen voltage relate to the OT impedance?

OT impedance and B+ determine the load line, and screen voltage determines the location of the Vg1=0 knee on the Ia vs. Va plot for the tube. The load line should pass through or a bit below the knee, so a good match for Zaa, Va, and Vg2 is desirable. See http://tubedata.milbert.com/sheets/093/6/6L6GC.pdf , top of pg. 6. Pa max along the load line is also important. That is easily estimated as Va^2/Zaa = 500^2/2300 = 109. You have 2x 30W on each side, so 109/60 = 180%. For push-pull something less than 200% should be fine.
dorrisant wrote:I feel that sometimes someone has to be the one to ask those questions in an intelligent way for the sake of all the ones that are too hesitant to do so.
People asking questions is the reason other people are here!
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