Modded 1966 Twin UL OT

[Opry Audio]

Hey y’all,

I’m working on a modded 1966 Fender Twin. The PI has been changed to a cathodyne and a Master volume has been installed on the back. From what I can tell it works well! I just went through and did a full recap, the amp sounds pretty nice. Onto the real question: it has a replaced UL output transformer running 2 6L6’s. Someone has installed a GZ34 rectifier in one of the unused sockets.

2 8 ohm speakers are installed but I’m wondering how to determine if it’s looking for an 8ohm load total. Is it something that ultimately does not matter much if the bias is within normal range?

Another thing I’m wondering is if this GZ34 can really handle all the current needed for this amp. The PT is replaced as well. The work done on it looks really great which is a good sign that someone has thought out all of these things, but I have to know for myself before returning it to its owner. Thanks!

[2deaf]

Got any numbers on those transformers?  What is the B+ voltage with the standby switch closed?

[PRR]

What did it come in for? Minor accident or major burn-down?

[shooter]

 

[pdf64]

I wonder if a cathodyne might not have sufficient output to fully drive 6L6 in a UL arrangement?
On the face of things, I think it unlikely that a GZ34 would have a problem with such a loading.

[shooter]

might be close, my last cathodyne got 23.1Vac rms before is started to distort.  That's with ~ 150vdc plate and ~ 125vdc cathode.

[tubeswell]

Is there a problem with the amp?

[Joel in Texas]

Opry, if you just want to know if the speaker load is “appropriate” for this OT, I agree with 2deaf’s suggestion as the best starting point - look for manufacturer name and any codes stamped on the OT, and google it up. Report back here with that info if nothing turns up. Someone here might have docs or search strategies that go beyond ye olde google machine.

Option 2 is to feed the OT primaries (out of circuit, no connections) some known, preferably lower range, precisely measured AC like 5.7 volts from the rectifier heater tap or other measurable AC source, and then precisely measure AC on the (out of circuit, no connections) OT secondary.  Once you have that info you can calculate the turns ratio, and also get the primary impedance for a given speaker load.

See this: http://www.sarris.info/main/calculating-output-transformer-impedance

Report back here with what you find if you go this route, and people can tell you if what would be reflected on the primary side seems good for two 6l6s, given an 8 ohm load.

[Opry Audio]

Hey guys! Thanks for the replies regarding the B+, it’s higher than normal for a twin. As follows.

B+
A - 561v
B - 476v
C - 463v
D - 423.8v

When it came in it only clearly needed a recap, no failures to speak of. The amp sounds extremely good, there’s a bit of a residual hum that I haven’t scoped yet. I’m very sorry y’all, this is a LTP after all... lots of sprague orange drops and goop threw me off.

Back to my main question, the OT is a dynaco A- 431. 60w OT with a UL tap. I’m sure someone here is familiar

[AmberB]

It sounds like whoever did the mod wanted an amp that stayed clean at higher volume...

[Opry Audio]

It does stay really clean, it has a PPIMV installed where the ext speaker jack was so it breaks up really nicely on 10 without blowing your ears out.

Here’s some pics for anyone that’s interested

[Joel in Texas]

When I google Dynaco A-431, several postings suggest a primary impedance of 4.3K ohms, with secondaries for 4, 8, and 16 ohms.  4.3k is definitely in the "normal" range for a pair of 6L6 tubes. 

But the next logical questions are, how are the two 8-ohm'ers wired (series=16? or parallel=4?), and what secondary wire might be hooked up to the speakers? 

Can you see multiple secondary wires?  Any chance you see Black, Brown, Orange and Yellow for secondary wires?
One posting for a pair of Dynaco A-431s says: "Secondary G (Black) to 4 Ohm (Brown) 8 Ohm (Orange) 16 Ohm (Yellow)".  I can't vouch for accuracy.  It's from the internet after all.  And I have no idea how many versions of the A-431 might exist, but if I had to bet I would guess this is pretty standard.

So if you see a Brown wire connected to a 4 ohm load, or a Yellow wire connected to a 16 ohm load, there's a good chance everything is all lined up as it should be.  [Edit - Just seeing your gut shots now, but I can't tell for sure: is it Black and Orange going to the speaker jack? And maybe Brown and Yellow are tied off with shrink tube?]

If this "informed guess work" doesn't satisfy, again you could feed the primary some AC, measure AC on the secondaries, and do the calculations I mentioned at the link in my previous post.

[sluckey]

Dynaco A-431 = Sunn

[DummyLoad]

Hey guys! Thanks for the replies regarding the B+, it’s higher than normal for a twin. As follows.

B+
A - 561v
B - 476v
C - 463v
D - 423.8v

When it came in it only clearly needed a recap, no failures to speak of. The amp sounds extremely good, there’s a bit of a residual hum that I haven’t scoped yet. I’m very sorry y’all, this is a LTP after all... lots of sprague orange drops and goop threw me off.

Back to my main question, the OT is a dynaco A- 431. 60w OT with a UL tap. I’m sure someone here is familiar

guessing that PT is a ~800VCT 200mA unit: that would make 560V lightly loaded (deep AB1 idle). someone took a twin and converted to dynaco mkIII/sunn output stage. what a waste with 6L6GC in those holes. probably sound better w/ Kt88/6550. bias supply may be the issue there and why they ran 6L6GC, + maybe wanted less power? 

that bias PS looks like it needs some refreshing - suggest careful replacement of that selenium rectifier w/ a silicon type & replace those bias supply caps.

sunn & dynaco amps drove UL output stage with a cathodyne w/o issues with 500V B+ & slightly less B+ for cathodyne... 

--pete

[Opry Audio]

Hey guys! Thanks for the replies regarding the B+, it’s higher than normal for a twin. As follows.

B+
A - 561v
B - 476v
C - 463v
D - 423.8v

When it came in it only clearly needed a recap, no failures to speak of. The amp sounds extremely good, there’s a bit of a residual hum that I haven’t scoped yet. I’m very sorry y’all, this is a LTP after all... lots of sprague orange drops and goop threw me off.

Back to my main question, the OT is a dynaco A- 431. 60w OT with a UL tap. I’m sure someone here is familiar

guessing that PT is a ~800VCT 200mA unit: that would make 560V lightly loaded (deep AB1 idle). someone took a twin and converted to dynaco mkIII/sunn output stage. what a waste with 6L6GC in those holes. probably sound better w/ Kt88/6550. bias supply may be the issue there and why they ran 6L6GC, + maybe wanted less power? 

that bias PS looks like it needs some refreshing - suggest careful replacement of that selenium rectifier w/ a silicon type & replace those bias supply caps.

sunn & dynaco amps drove UL output stage with a cathodyne w/o issues with 500V B+ & slightly less B+ for cathodyne... 

--pete

Thanks, these are great suggestions. I just ordered 2 new sprague 50uf 150v. I think there is a bias issue. One of the tubes was only running -12mA at idle. Anyway, do you have any suggested replacements for this selenium rectifier? I’m not familiar with them. Would an IN4007 suffice?

[sluckey]

Would an IN4007 suffice?

Yes

[pdf64]

I suspect that awful selenium cap was used just because it could be bolted to the chassis. Because the bias board has disappeared.
Those bias caps are just dangling, true p2p style, which is somewhat  naff.
Hence I suggest to construct a new board for the bias supply.

[Opry Audio]

I suspect that awful selenium cap was used just because it could be bolted to the chassis. Because the bias board has disappeared.
Those bias caps are just dangling, true p2p style, which is somewhat  naff.
Hence I suggest to construct a new board for the bias supply.

Would a standard bias supply for a bassman or another PP 6L6 amp work?

[Opry Audio]

I believe the bias supply built is one that’s identical to the Sunn spectrum II

[pdf64]

I suspect that awful selenium cap was used just because it could be bolted to the chassis. Because the bias board has disappeared.
Those bias caps are just dangling, true p2p style, which is somewhat  naff.
Hence I suggest to construct a new board for the bias supply.

Would a standard bias supply for a bassman or another PP 6L6 amp work?

Why not use the current circuit? With perhaps a tweak to get it a better range.
Have a look at Merlin’s Valve Wizard site for different ideas.
Without being sure of the specifics of the PT’s windings, it’s tricky to advise any further.

[Opry Audio]

I suspect that awful selenium cap was used just because it could be bolted to the chassis. Because the bias board has disappeared.
Those bias caps are just dangling, true p2p style, which is somewhat  naff.
Hence I suggest to construct a new board for the bias supply.

Would a standard bias supply for a bassman or another PP 6L6 amp work?

Why not use the current circuit? With perhaps a tweak to get it a better range.
Have a look at Merlin’s Valve Wizard site for different ideas.
Without being sure of the specifics of the PT’s windings, it’s tricky to advise any further.

Makes total sense. I unwired the selenium rectifier and used an IN4007. These Mallory caps are old but they test well. 60uf each and both have an ESR of .9R. I had ordered these from CE but they’re back ordered apparently. I have each of these tubes biased at 39mA and just monitoring now to see if anything comes up.

[pdf64]

The residual hum may be due to an inadequately filtered HT; screen grids need a smoother supply than the plates, but UL forces them to share the same supply node.
UL amps that have a good rep generally use CLC (Pi) filtered supply to the OT CT / plates / screen grids.

[PRR]

I guess my point was: If it has run OK long enough to need a re-cap, there's nothing which urgently needs checking/fixing.

Yes, look and sniff for leaks, scorch-marks, bulges.

Yes, the Selenium is a known time-bomb and for a "re-cap" I would just call it part of the job. Aside from possible melt-down, it stinks bad when it blows.

But since it is not "stock" and no authentic documentation was provided, I would just give 30 days parts/labor with NO promises on anything else. It may be better than factory, it may be about to die, and there's no practical or sure way to know.

[tubeswell]

Looking at the pics you posted above, the only thing I'd note in addition to what everyone has said so far, is the PT bolts don't look that secure, and if you have the tools, you might want to make up a little plate of 2mm ally or steel (with holes pushed for the winding leads to pass through) to cover the old hole and enable more secure holes to be drilled for the PT mounting bolts. YMMV


Also, if there's residual hum, and you still haven't replaced the bias supply caps, then replace those (even if they 'test' okay).

[Joel in Texas]

And also, back to your original question, regarding possible impedance mis-match:

Can you see multiple secondary wires?  Any chance you see Black, Brown, Orange and Yellow for secondary wires?
One posting for a pair of Dynaco A-431s says: "Secondary G (Black) to 4 Ohm (Brown) 8 Ohm (Orange) 16 Ohm (Yellow)".  I can't vouch for accuracy.  It's from the internet after all.  And I have no idea how many versions of the A-431 might exist, but if I had to bet I would guess this is pretty standard.

So if you see a Brown wire connected to a 4 ohm load, or a Yellow wire connected to a 16 ohm load, there's a good chance everything is all lined up as it should be. 

[Edit - Just seeing your gut shots now, but I can't tell for sure: is it Black and Orange going to the speaker jack? And maybe Brown and Yellow are tied off with shrink tube?]

If my interpretation of the pics you posted is correct, and you have Black and Orange secondaries going to the speaker jack, and the customer had two 8-ohm speakers as you stated, then there is a decent chance it has been running on a mis-matched load, which you might be able to address by switching to either Black and Brown (4-ohm) or Black and Yellow (16-ohm).

I'm not an impedance-matching purist. It can be OK to run a 4-ohm or 16-ohm load on an 8-ohm tap, etc. I'm just getting back to your original question, which was whether or not you had appropriate impedance-matching in this modified amp.  And based on the above info and the gut-shot you provided, it seems possible that the current setup is mismatched, and that you could possibly address that by switching to a more appropriate secondary wiring to the output jack.

[Opry Audio]

Yes! I have changed the secondary tap to 4ohm :) thanks for all the knowledge of dynaco OT’s here. I will be recapping the bias supply on Monday when the caps get here and this amp should be right as rain. The guy who brought this to me only paid $800 for this amp so I’d say he’s getting a pretty good deal even with the mods. He’s really lucky, these are overall very well done. The MV works extremely well. It’s definitely not a twin but what it is on it’s own is pretty cool. From the looks of the components it’s likely that it was modded in the early 70’s maybe? Before the caps went south. I’ve gotten a few other fenders modded by whoever this was, and they were all cool. I wish I knew who it was. I presume he was local and these amps stayed in the area.

[Opry Audio]

So, I completed the servicing aspect of this amp. I rebuilt the bias section with a silicon diode and spragues. This amp is a little too clean. Almost sterile sounding. Not really a lot of punch or dynamics to it.

The plate and screen supply is pretty near 500v. I have each tube biased at 40ma, which from what I understand about AB1 biasing, is 70%.

There is also still a hum while the tubes first start conducting and then goes away. Maybe having something to do with the rectifier? Any advice for finding this and making this amp sound a little better would be appreciated :) thanks for all your help already 

[Opry Audio]

Also, just as a note here.. I noticed that the 8 ohm tap sounded much bette than the 4 ohm tap, so I may revert back and see if that’s the factor that changed from good to not so good.

[pdf64]

Try it without the global negative feedback, eg lift a leg of the 820 series feedback resistor.
As UL is already local negative feedback that will be linearising the output.
As will idling the 6L6 at 40mA; try bringing that down, it seems a waste of tube life to idle them at 20W in fixed bias.

[Opry Audio]

Try it without the global negative feedback, eg lift a leg of the 820 series feedback resistor.
As UL is already local negative feedback that will be linearising the output.
As will idling the 6L6 at 40mA; try bringing that down, it seems a waste of tube life to idle them at 20W in fixed bias.

I don’t believe there is a NFB resistor in here but maybe I’m not looking in the right place? I have the tubes biased back down to 25mA each. Sometimes it creeps up to 27 after 20 minutes or so. It feels like the amp is not as loud as it should be.

I still cannot track down this hum. It’s not coming up when I scope the output at all. There basically are no caps left to replace in terms of filtering

[sluckey]

I don’t believe there is a NFB resistor in here but maybe I’m not looking in the right place?

It's on the schematic.

[shooter]

It’s not coming up when I scope the output

at the speaker?
IF you can hear it but not see it, it's not there 

[2deaf]

I don’t believe there is a NFB resistor in here but maybe I’m not looking in the right place?

Look between the siliconed orange caps and the blue molded cap.  It looks like it is underneath a disc cap and it connects to the 100 ohm resistor that elevates the LTP.  There is an orange wire connected to the 100 ohm resistor that in all likely hood connects to a screen tap on the OT.  Leave that orange wire connected to the 100 ohm resistor.

[Opry Audio]

Hey y’all, I feel like such a dummy. The hum is coming from the screen supply due to insufficient filtering as was stated before. I’m gonna replace the reservoir capacitors with 2 100uf 350v. Will this be too much for the rectifier? The originals were 70x70

[Opry Audio]

There is a GZ34 btw

[Opry Audio]

Just answered my own question.. I forgot about caps in series.. anyway, I replaced with 100uf and the hum is still there, and doesn’t sound very good.. any tips are appreciated if you can think of anything!

[DummyLoad]

how is the PS wired for the output stage?  is there a CLC (pi filter) hanging off the GZ34? if not, you NEED clean power to the plates/screens with UL. the original Dynaco/Sunn plans have a CLC filter: GZ34 > 30UF > 1.75H@200mA > 20uF: see attached and duplicate that PS from the PI supply back to the output tubes, if you leave the fender choke in place for the preamps, that's fine too, but you may need the chassis space for the B+ "A" node CLC network. 


--pete.   

[Opry Audio]

how is the PS wired for the output stage?  is there a CLC (pi filter) hanging off the GZ34? if not, you NEED clean power to the plates/screens with UL. the original Dynaco/Sunn plans have a CLC filter: GZ34 > 30UF > 1.75H@200mA > 20uF: see attached and duplicate that PS from the PI supply back to the output tubes, if you leave the fender choke in place for the preamps, that's fine too, but you may need the chassis space for the B+ "A" node CLC network. 


--pete.

Interesting. Yeah, the power supply in this amp is the stock twin reverb nodes. No pi filter. So, is installing another choke the only way to get sufficient filtering for the node “A”? Any possibilities that might be easy to implement. It’s becoming harder to assume that these mods had worked really well at one point. Someone had obviously played it for years.

In my soul part of me wants this client to let me bring it back to stock.. but I want to see if this arrangement can be good.

[2deaf]

The hum is coming from the screen supply due to insufficient filtering as was stated before.

This amplifier doesn't have a screen supply.  What used to be the screen supply is now just the junction of the choke and a 6.2K resistor.  The other end of the 6.2K resistor is the P.I. node.  The junction of the choke and the 6.2K resistor (formerly the screen supply node) has no filtering.  The P.I. node does have filtering.   

Does the hum persist with the MV turned clear down?

Does the hum persist with the standby switch open?

If you do go with a CLC filter, the Hammond 158T will bolt on with no modifications.  The original choke would not be used in this case.

[DummyLoad]

no disrespect to 2deaf, but with 500V+ supply, i'd punch the extra hole and plant a 159T: it's a 500V part & the 158T is a 400V part. will the 158T work? yes, just peace of mind.


--pete

[2deaf]

no disrespect to 2deaf, but with 500V+ supply, i'd punch the extra hole and plant a 159T: it's a 500V part & the 158T is a 400V part. will the 158T work? yes, just peace of mind.

There might be a little more involved than just punching an extra hole.  The tube socket looks to be in the way if you try to move the hole 3/4" that way and the bias pot won't allow a hole any further in that direction.

The CLC filter is going to require capacitors that can handle over 500V on each end of the choke.  Just something to keep in mind.   

[Opry Audio]

Hmm, I think I would be more inclined to get him to let me put some nice fender twin iron in there and bring it back to stock. I think it would take me a similar amount of time vs installing a CLC filter that’s going to be tricky to fit in there, with no guarantee that it’s gonna be good at the end of it.

Hypothetically, is there any way this could have worked with just node A supplying the plates and screens? Or most likely a failed experiment?

[DummyLoad]

returning to stock would be wise.

doug had/has? some scratch and dent twin power transformers. he made a good deal on them. hit him up.

build the AB763.

i'd be interested in that UL iron if the price were to be fair.

i was curious to see if i could make a hammond CH14 size choke work. i have a fender bassman 100 that i believe shares the same chassis dimensions as the 70's twin, the OT is much larger than that of the twin. i was in the process of converting to a AB165 plan but shelved it. out of curiosity, i test fit a CH14 size choke in the chassis. what i did was enlarge the stock hole closest to the front, and with a couple of #10 fender washers used the bias pot hole for the other end of the choke, then i measured 1.125 inches up on the same X plane of the old bias pot hole and punched a .375. hole for bias pot new location. i am using #8 fasteners for the choke. see pics in links.

https://www.dropbox.com/s/6lz30ajdfq6tvgp/20201024_212948.jpg?dl=0

https://www.dropbox.com/s/dehhja7edja4qp6/20201024_213012.jpg?dl=0

https://www.dropbox.com/s/bl1de7mthingv5k/20201024_213905.jpg?dl=0


--pete

[2deaf]

Hypothetically, is there any way this could have worked with just node A supplying the plates and screens? Or most likely a failed experiment?

Here's an idea.  Cap's fit in the doghouse.  Just kidding--the guy would be pissed if he found out.

[Opry Audio]

The hum is coming from the screen supply due to insufficient filtering as was stated before.

This amplifier doesn't have a screen supply.  What used to be the screen supply is now just the junction of the choke and a 6.2K resistor.  The other end of the 6.2K resistor is the P.I. node.  The junction of the choke and the 6.2K resistor (formerly the screen supply node) has no filtering.  The P.I. node does have filtering.   

Does the hum persist with the MV turned clear down?

Does the hum persist with the standby switch open?

If you do go with a CLC filter, the Hammond 158T will bolt on with no modifications.  The original choke would not be used in this case.

Hey 2deaf sorry I miss your question here: yes, the hum goes away when the master volume is turned down and standby open. It’s present with all tubes pulled besides 6L6’s and rectifier

[Opry Audio]

I got this guy to agree to let me bring it back to stock.. thank goodness. I honestly have had so many crazy weird and super difficult repairs recently so I’m relieved to just rebuild half of a twin instead of going further down this road.. haha. That being said if anyone has recs for great Twin reverb transformers I am all ears.

[DummyLoad]

https://www.hawkusa.com/manufacturers/hammond-mfg/enclosures/290fx

https://www.hawkusa.com/manufacturers/hammond-mfg/enclosures/1750w


hawk electronics seem have the best prices on hammond iron. depending on stock, sometimes hammond has to drop-ship for them, so don't be in a too much of a rush as that usually delays the order 1-2 business days.


--pete

[Opry Audio]

https://www.hawkusa.com/manufacturers/hammond-mfg/enclosures/290fx

https://www.hawkusa.com/manufacturers/hammond-mfg/enclosures/1750w


hawk electronics seem have the best prices on hammond iron. depending on stock, sometimes hammond has to drop-ship for them, so don't be in a too much of a rush as that usually delays the order 1-2 business days.


--pete

Thanks! Any idea how this measures up to the stock TR OT? This guy wants the nicest possible iron I can put in it i think. 

[DummyLoad]

https://www.hawkusa.com/manufacturers/hammond-mfg/enclosures/290fx

https://www.hawkusa.com/manufacturers/hammond-mfg/enclosures/1750w

hawk electronics seem have the best prices on hammond iron. depending on stock, sometimes hammond has to drop-ship for them, so don't be in a too much of a rush as that usually delays the order 1-2 business days.

--pete

Thanks! Any idea how this measures up to the stock TR OT? This guy wants the nicest possible iron I can put in it i think.

i have used both the PT and OT for repairs & experiments. the output delivers exceptional tone for the money. i have also breadboarded several amps with those parts. IMO, they sound as good as the 60's stuff.

--pete