Blackfacing a 135W ultralinear Twin Reverb?

[Chuck H]

Or... If he does any demo work he could lash it to a chain off a crane and swing it!!!

[Shane]

When I've modded these in the past I have generally removed the negative feedback (UL amps dont really need it), Upped the screen resistors to 1K min, 2K2 for preference as per pdf64's reply, and finally convert one tube per side to run triode mode. Yes you lose a few watts but it warms them up a fair bit.

I also mod the normal channel so the bright switch acts as a tone stack lift and change the anode and cathode resistor values to be more like Dumble High plate values.
Oh and also change it so you get reverb on both channels.

Cheers
Shane

[bob p]

Disconnecting the NFB loop without re-scaling the gain of the output stage would result in a pretty dramatic gain change for tthe output stage.
Running a pair of tubes in triode mode will result in a pretty dramatic decrease in headroom for the output stage.
Lifting the tonestack will result in fairly dramatic changes in preamp gain.
To my ears, those changes go beyond blackfacing. I think that "warms them up a fair bit" could be an understatement.

[Steve Conner]

Running one tube per side in triode mode is an interesting idea, kind of like Mesa's output stage that uses both 6L6s and EL34s. It would probably reduce screen dissipation in all of the tubes.

[Tom Phillips]

Running one tube per side in triode mode is an interesting idea...

I did that for once for a customer after I explained that his 135W UL Twin could not be simply "Blackfaced." The partial triode mode mod did tame the amp down and the customer was happy with the result. It was many years ago so I don't remember what else I may have done to the amp. I got that partial triode mod idea from one of Kevin O'Connor's TUT books.

[bob p]

I have one of those Mesa amps that has the multi-way output stage, with switchable tubes (6L6, EL34 and 6V6), B+ reduction, Class-A/Simulclass and the half triode thing. Some of the settings are gimmicky and aren't all that useful. I think that the EL34 + 6L6 combination is interesting, and the SimulClass thing is interesting when I've got a set of 6L6 in the amp. But I have to admit I don't use the triode mode all that often, and the effect of B+ switching isn't anything to write home about. YMMV.

[olddawg]

How about removing the 2 outside tubes and installing a pair of 6v6 yellow jackets?

[J M Fahey]

Cool.
That coupled to the 510V supply will put those 6V6 in orbit at close to light speed.
Just what's needed to bring down those pesky North Korean missiles

[olddawg]

Cool.
That coupled to the 510V supply will put those 6V6 in orbit at close to light speed.
Just what's needed to bring down those pesky North Korean missiles

I was half serious. Not sure if Yellow Jackets are compensated or not. I have known them to be used in TRs before. Although I don't know why. It would be simpler and more economical just to get a smaller, lighter amp.

[J M Fahey]

I was half serious.

Same here

[Chuck H]

I think it's a specific tube thing. Some new 6V6's will take a lot more voltage than the old ones. The JJ's come to mind. They spec at max 500Vp. I haven't tried them but I know they have been used in TR's. Maybe lift the screens from the UL taps and take the screen voltage from the series filter junction? I know this has been poo poo'd but I don't know that I would put over 500V on both the plates AND screens of any tube designated "6V6". Not if I were planning to overdrive it anyway. Otherwise, you could just tack on another PS node (two caps and a resistor) just for the screens.

[bob p]

Funny that you mention this, Chuck. I took a lot of heat from the guy who sells scumback speakers for saying exactly the same thing. He insisted that only NOS Mazda 6L6 would handle today's voltages in amps like the DR, and called me an idiot for suggesting that the new JJ 6V6 would fit the bill nicely. I guess he just likes buying NOS tubes.

I agree, the biggest concern would be about screen dissipation. IME if you properly limit the screen dissipation then new production JJ 6V6 can handle voltages like that just fine.

[imaradiostar]

I agree with the above assessment about lowering screen supply.

Doesn't the ultralinear twin have a center tapped bridge in the B+ supply? I just double-checked, one could build a "screen" node at half of B+ from the junction of the two 220uF caps, at the red/yellow wire. Filter that through a small dropping resistor and a 50Uf cap and you'll be all set. It could make a nice screen supply to run el34's or 6v6's in the amp.

The other option is to use a cheap "VVR" circuit to drop the main B+ to some fraction of the plate voltage. I have an amp built this way with a simple mosfet circuit that allows me to adjust the screens to between 100% and 50% of B+.

jamie

[Leo_Gnardo]

I agree with the above assessment about lowering screen supply.

Doesn't the ultralinear twin have a center tapped bridge in the B+ supply? I just double-checked, one could build a "screen" node at half of B+ from the junction of the two 220uF caps, at the red/yellow wire. Filter that through a small dropping resistor and a 50Uf cap and you'll be all set. It could make a nice screen supply to run el34's or 6v6's in the amp.

That halfway point @ the junction of the series filter caps looks tempting BUT I've tried it & it sags like mad, also upsetting the voltage balance between the series caps. Enough to force the voltage beyond rated 300V for the "upper" cap. So much for that.

Just finished taming a Traynor YBA-1A MkII with 550V B+. Owner wants to use KT88. So far so good. But with screen grids run off the usual post-choke HV point, they're beyond the safe point & threaten tube wreckage. Here's what I did, simple 4K 10W resistor in series with the choke. Now the SG supply is more like 450V and grids aren't drawing worrisome current. Amp delivers a clean 65W to either 8 or 16 ohms and - doesn't have the usual brrrr 120Hz modulation of notes played loud. Give it a try, all you need is a 10W resistor, say 3000-5000 ohms. You might need even more than that, just monitor your SG current 'til you find a happy medium for the tubes you want to use.

I've had bad experiences fixing amps where owners have plugged in Yellow Jackets or other output tube adapters. Maybe they would work in an amp that had 400V or less B+. But I've seen too many smoked YJ's & disappointed buyers with their hopes (and wallets) dashed once again.

[imaradiostar]

That halfway point @ the junction of the series filter caps looks tempting BUT I've tried it & it sags like mad, also upsetting the voltage balance between the series caps. Enough to force the voltage beyond rated 300V for the "upper" cap. So much for that.

Have you tried it on one of these amps specifically?

It really depends on the design of the amp. This amp is designed as essentially a full wave bridge with a center tap connected to the series caps after the rectifiers- it shouldn't sag much if at all. I've done it on similar designs and not had trouble. If a given amp uses series caps without the transformer center tap present, yes, it will sag like crazy.

jamie