Twin Reverb [UL]- rebuild help.

[Sea Chief]

Well, unless the amp has other issues related to a conductive board, tonal or safety, you could also rebuild the bias circuit on it's own wee board and elevate any connections from the PI coupling caps to the power tube grids. Not pretty, but it would probably solve the problem and it's a lot easier (and cheaper) than replacing the whole circuit board.

But doesn't half of the bias circuit already sit on the separate 'rectifier board'? so you mean separating off the last 3" or so of the m'board, where the two fat 0.1's sit?

[Sea Chief]

https://tubeamplifierparts.com/turre...=0&sort=normal

Here's a source for AB763 circuit boards... Best of luck.

Thanks for this info Tqi. I might be lucky & Rift may help me out on the R's needed. As to wire, from experience the only stuff I'd use is solid core cloth as it stays put.. saves much time & effort.

On a separate note, speakers: if I measure 3.5 ohms (in the Twin/ in series iirc) each, & no obvious signs of VC rubbing.. can I conclude they are healthy?

[pdf64]

An option would be to solder a suitable lengths of tagstrip on to the existing eyelets, then that gives you spare tags that are above and isolated from the existing board to mount everything that's in the dc circuit of the power tube g1s.
Obviously the tags soldered to the eyelets don't get connected to anything else.
Not pretty but if done properly should be functional and would save an awful lot of work.

The best test for speakers is to play a 'known good' amp through them.
I think your 135W TR speakers should be wired in parallel http://schems.com/schematicheaven.ne..._135_schem.pdf

[J M Fahey]

On a separate note, speakers: if I measure 3.5 ohms (in the Twin/ in series iirc) each, & no obvious signs of VC rubbing.. can I conclude they are healthy?

They are in parallel and you seem to be measuring them while still interconnected so you are not measuring them "each" but the whole shebang.

You may measure across 1 speaker terminals, then across the other .... not surprisingly you get same value , and if you test it, you will find you also have a 3.5 ohm plug at the other end of the short speaker cable.

[Chuck H]

Here's the schematic BTW.

[Chuck H]

I mean, build the entire bias supply on a separate board and run the PT bias lead to that board. It shouldn't matter where it is in the amp now. Build the whole thing on another board regardless of where any components are now. Then elevate the entire grid feed circuit to the power tubes from the board from the downstream side of the PI coupling caps all the way to the power tube grid pins. I like Pete's idea soldering terminal strips to the (now unused) eyelets.

It sounds complicated, but it's really not. There are only a handful of components involved. You could even propose this to Rift. They would get it without question. Though they may also take the position that if the board is bad then the board should be replaced (along with nearly EVERY COMPONENT ON THE BOARD now $$$).

[Sea Chief]

Though they may also take the position that if the board is bad then the board should be replaced (along with nearly EVERY COMPONENT ON THE BOARD now $$$).

But this is what they have concluded, as I said initially. I don't need replace many components surely if I keep all the big blue caps/ the new bias area caps & R's/ the new 25/25 pre caps, roach & any orig R's that measure ok I assume too. That's the majority of the board's components kept & re-used.

I'm not quite sure, if the board is said to be conductive, why the ideas being proposed of splitting off the bias area into a new board. I mea the remaining board will still be conductive, won't it?

[Sea Chief]

In situations such as this it is prudent to try another set of power tubes. One can never be 100% sure that new parts are good parts. New tubes can have defects. One tube within the quad can have an intermittent problem that loads down the bias voltage and cause a runaway condition.

Yes understood, but Rift wouldn't have -not- checked this, & possibly sent me back any of the new 4 I bought faulty. They would check new tubes on their tester before installing; & once tested ok, make sure they don't ruin any & so as a customer I am saddled with any of the 4 having been made bad by them even after extensive amp tests they've done. That would be v. bad practise & in a month of sundays I'd not expect from such a reputable Co. Tbh I wouldn't even dare ask if the tubes I've rcvd back are ok, its just rude.

[J M Fahey]

"Rift" has been mentioned **thirteen* times in this (so far) short thread, usually accompanied by Praise Words such as "Pro", "a favour to me", "would have done every possible test to pinpoint if they could", "Knowing their [his] huge wealth of knowledge as being both an excellent builder & as one of the best repairer both here in UK & in the EU", "I just take his word", "its had a tip to toe service/ everything plus the kitchen sink has been thrown at it", "they've (pros) tried every little thing", "flying off @ Rift! " , "Rift wouldn't have -not- checked this", "such a reputable Co", "I wouldn't even dare ask if the tubes I've rcvd back are ok, its just rude. "

Makes me ask, sincerely, is this a true doubt or a Rift Infomercial?
Because it certainly sounds like one.

And makes me wonder: given your trust and adoration, why don´t you let them solve your problem?.

In this case money is not a problem.

In fact it´s way better to invest some extra money letting them do their magic, whatever they require, than risking a possibly botched self repair.
At least Logic seems to indicate that path.

[Chuck H]

But this is what they have concluded, as I said initially. I don't need replace many components surely if I keep all the big blue caps/ the new bias area caps & R's/ the new 25/25 pre caps, roach & any orig R's that measure ok I assume too. That's the majority of the board's components kept & re-used.

I'm not quite sure, if the board is said to be conductive, why the ideas being proposed of splitting off the bias area into a new board. I mea the remaining board will still be conductive, won't it?

Yes, the remaining is conductive. The ideal move is to replace it, BUT... If the only problem is the bias issue it could be fixed by the method I suggested. I offered it because sometimes cost is a factor and you seemed to acknowledge that. How you proceed is up to you. Since you wouldn't be doing the work yourself it's very expensive. Since "I" do the work myself it's just a shoulder slump and and a sigh. I've rebuilt many boards that were originally done on that black paper crap. I would guess that I've built as many boards to replace those black paper boards as I have to make original designs. So I'm not a proponent of keeping the board. I'm just proposing a potentially more economical solution IF it works and IF it suits your budget and circumstances.

[Sea Chief]

Ok understand a bit more having read thru again Chuck.. thanks for the suggestion.

Can I definitely establish then, that having this 'run away bias' issue (which I do not fully understand tbh), can be isolated by a definitive 'dividing line' on the main board, ie separating off the bias components? s'one here pointed @ the trem circuit -poss- a culprit too.

Could you help me with a few very brief Q's I could ask Rift to establish what has been tested on the board re. this issue?

I think then a planA: try & physically separate the board in situ, create a new bias/ end board (once I establish exactly which bits are which here/ where to cut) & swap over the components, most of which are new by Rift.

[Sea Chief]

jmfavy- ignored/ dont see yr stuff. prob insults so yr wasting yr time

[Sea Chief]

Just spoke to Rift with all your suggestions: all of which he said VG ideas. For this ULTR its better to do this he said tho:

as its had some dog's dinner mods done (it had, but thought Rift had undone, & board now totally stock) before I got it, which aren't easy for me to spot so thought it was back orig/ apparantly still bits here & there on board.. & as there is no UL135 layout out there for me to follow if I were to try replicate the stock amp back again- Rift proposes this planB:

Rebuild to AB763 using a std repro board, just refettling the odd thing to suit like the OT tap he mentioned. So then it'll be down to ~ 100w (better for my pair of jensen C12Q spkrs), & will actually suit the BF repro faceplate it has on it (the MV long since awol).

I mentioned -all- the possible Q's as to double-checking this idea to Rift, all of which answered convincingly that this is the most sensible route -for me, with this amp- to take.

Opinions please.

[Chuck H]

Many people think that "UL" is some sort of ultra efficient power circuit that will make a regular tube a super tube. Not so. You may not have 135W now (probably not in fact) and you would likely have similar power with the tubes wired as they are in an AB763 circuit. Maybe a few watts less due to lower screen volts. This would also add the expense of of a choke to the circuit. At face value it may seem like changing to the simpler AB763 circuit is the easier way to go, but it would involve physical changes to the amp for retrofitting parts and design change considerations for differences in the transformers. The up side is that it's a popular enough route that there should be a bit of on line info and discussion to follow.

[66 Kicks]

I've never considered the power section on the various AB763 circuits to be a defining characteristic. If it were, I guess you would have to get a new power transformer so that you could run a conventional full-wave rectifier instead of the bridge rectifier.

The screen grid taps don't have anything to do with the main board or any other board -- they go directly to the power tubes.

The original pre-amp circuit on the 135W is really, really close to the AB763. If you put a new AB763 board in, you could just follow the standard AB763 layout. The master volume and the boost switch apparently have already been removed, anyway.

The other two boards should be replaced if board conduction is a legitimate concern.