63 Ampeg SET

[shooter]

I've been back n forth with this customer since 2014.  He finally brought it by, I might wish he hadn't

He went from complete restore to, just fix it, which is all I tentatively agreed to once I pointed out all the rust, rust dust, cobbled face-plate, etc.   I re-drew the original schematic, pretty sure it's *close*

Should be an interesting *challenge*

EDIT: found a way better schematic, thx Doug
http://el34world.com/charts/Schematics/files/ampeg/Ampeg_et2b_superechotwn.pdf

[Paul1453]

This was yours and you sold it to him or,

He bought it somewhere else and brought it to you to fix for him.

I don't know why you'd really want stereo in a guitar amp.   

If it had been my amp, I'd probably take it apart to make 2 amps out of it. 

[shooter]

I woulda only sold something like that as spare parts!

brought it up, no tubes, after ohming and measuring cap-cans, B+ unloaded both sides about 412VDC, AC ripple Lside 200mV, Rside 140mV.  Found Lside 22k dropping R at 10k, heat damaged, so gonna swap that.

Hopefully test one side pre-amp as a quick *tube-tester*, need to ohm chk OT's, kinda nice having 2 of everything.

The rust is goin to be a pain, stiff brushes, alcohol, micro fittings on the shopvac

[HotBluePlates]

He went from complete restore to, just fix it, which is all I tentatively agreed to once I pointed out all the rust, rust dust, cobbled face-plate, etc. ...

...The rust is goin to be a pain, stiff brushes, alcohol, micro fittings on the shopvac

Why are you going to bother cleaning rust when the customer says, "Just fix it"?

[shooter]

Why are you going to bother

I'm only working on the *crusty* stuff, the stained, stabilized stuff I'm leaving.  I won't loose sleep if rust migrates to the HV and causes a fire, but If I get some and issue a stern warning, my *ethical* side will be happier

There are 2 oddities-mods I've found, the one, 10k dropping R in the power rail appears to actually be 10k not 22K heat damaged to 10K.  It provides the tap for the Lside PI, possibly for more drive?

The other *mod*, on the trem grid,(V5A-1), original shows 270k to ground, there is now 2 270's in series to ground, done to *tame* or *boost* trem?

[sluckey]

There are 2 oddities-mods I've found, the one, 10k dropping R in the power rail appears to actually be 10k not 22K heat damaged to 10K.  It provides the tap for the Lside PI, possibly for more drive?

The load is different for those B+ nodes so it seems reasonable that the resistors would be different values. One of them has to also supply the reverb circuit. Does the resistor in question look original?

The other *mod*, on the trem grid,(V5A-1), original shows 270k to ground, there is now 2 270's in series to ground, done to *tame* or *boost* trem?

I think Joe Piazza just overlooked that resistor. The original Ampeg schematic shows two 270Ks.

[shooter]

The original Ampeg schematic

I gotta get the original, the one I grabbed was the ET-1 recto version.  Joe did a great job though!
Yes, it does look original, thanks for the help.

more findings, some confusion;

started on Rside, 2 PA tubes,(calling them 1st pair), got these #'s;
Vp 382
Vk 14.5
Vg2 365
Math yields ~ 19W/tube  ~ 53mA/tube
left it on ~ 5min

Moved to Lside, same tubes, about same #'s
Back to Rside, one original, one new(orig, untested) tube,
Vk dropped to 12.8, but Vp stayed about the same.
Next orig untested tube;
Vp about the same, Vk about 13.8

now for the *odd*;
put the *2nd* pair in Lside, nothing in R
at about the 1 min mark one tube flashed over internally, Vk started rising, killed pwr at about Vk = 15.3
put *1st pair* back in Rside, nothing L, No flashover, but about the 1 min point, Vk started rising, Vp didn't seem to care

I was gonna temp in 2 390r(closest I have on hand), so each tube has it's own Rk and see if the "issues" are tube or PS

[shooter]

Tube test results;
temp'd in a 390r and 47uF for each tube - used Rside PS for now;

Plate volts 387  tube type JJ7591S  Max plate 19W

Tube 1  16.6vdc   42.6mA   15.8Wpdis
tube 2   14.5vdc   37.2mA   13.9Wpdis
tube 3   16.6vdc   42.6mA   15.8Wpdis
tube 4   15.5vdc   39.7mA   14.8Wpdis

tested for 5min each pair, calling them *good* enough to move on for now.
my guess is the original cathode Ecap is dying, causing "I" to rise to the occasion?
(it's part of the cap-can, which I'm recommending be replaced)

After work I'll run the same on the Lside PS

[HotBluePlates]

temp'd in a 390r and 47uF for each tube ...
Plate volts 387  tube type JJ7591S  Max plate 19W

Tube 1  16.6vdc   42.6mA   15.8Wpdis
tube 2   14.5vdc   37.2mA   13.9Wpdis
tube 3   16.6vdc   42.6mA   15.8Wpdis
tube 4   15.5vdc   39.7mA   14.8Wpdis

tested for 5min each pair, calling them *good* enough to move on for now.
my guess is the original cathode Ecap is dying, causing "I" to rise to the occasion?
(it's part of the cap-can, which I'm recommending be replaced) ...

Are you talking about the 47µF bypass cap?

I'd suspect the old tubes themselves (one or more) had some issue causing excessive current. Because if the bypass cap was failing and leaking current, it would divert that current around the cathode resistor instead of through it, and drop the voltage across the cathode resistor. But if I read correctly, you noticed an issue with one tube which coincided with voltage across the cathode resistor rising. So that would seem to be a tube failure, not a cap failure.

Either way, a good ESR meter would tell you the health of the e-caps. But since everything seems to look good with all JJ 7591's, perhaps you just had a bum tube...

[shooter]

perhaps you just had a bum tube...

I suspect there are 1 or more, I will be doing a longer tube test, once I'm happy the Lside PS is good.  Here's a pic of how 1 tube came in, another (not shown) has minor residual *grayness* at  the window also.

Are you talking about the 47µF bypass cap?

Yes.  something I did hear as Vk was running UP, a very high pitched whine, increasing in pitch as volts increased.  I actually thought it was my tinnitus(ear ringing)
 

[shooter]

 
got one channel up and working!  I'm using the PA tubes shown in previous post, Amp is quiet, sounds good, jammed for about 10minutes.  Not messing with trem/verb yet.  Still running my temp'd in cathode r's n c's.  Next it's the *other* set of tubes in the same side, then on to the other Channel.

[shooter]

Need some insight if I could.  played all the tubes at least 30min on Rside, (one bad 6H9C).  So I moved one set to the Lside, and after 30sec to a minute I get lightning in the PA tubes, move them back to the Rside, all's fine.
The DCR on the OT, same on both, DC plate and Cathode close, till the arcing starts then the meters go nuts.
My gut factor says temp in the Rside OT, but before I unsolder, jumper over,  wanted to get some consensus.
The problem occurs whether the PI or Pre are in or out, no AC signals.  All the R values *touching* the PA tubes are within 10%.  Arcing definitely inside tube, not at socket.
Thx
   

[HotBluePlates]

Unless there's something I'm unaware of, an OT couldn't cause arcing in a tube with no applied signal. Which then makes me think a socket is the problem, or wiring to/from the socket.

[shooter]

an OT couldn't cause arcing

I'm in the straw drawing realm, my though was if internally the tranny was arcing over, it *might* cause High power reflected transients. I will re-tension-clean again, The solder connections are solid, but I'll reflow them.
Thx
 

[HotBluePlates]

Gotcha, and makes sense.

Well, trying the OT for the known-good side should tell the tale.

[shooter]

good side

I'm having 2nd thoughts on that, since it IS good, I like leaving good alone.  I don't have any PP OT's, but I have a 25w SE UL.  If the retention - cleaning doesn't work, i'll probably temp in the SE for a short test.

[HotBluePlates]

good side

I'm having 2nd thoughts on that, since it IS good, I like leaving good alone.  I don't have any PP OT's ...

The SE OT won't tell you much useful. You already have a same-OT on the side you've been testing with positive results, so it makes the most sense to use that OT to test the "bad side" as a means of ruling out an OT issue.

I'd look at it this way: If the "good OT" fails when put on the "bad side" it probably was ready to die anyway.

Or you might consider having at least one known good push-pull OT on hand for such testing in the future.

[shooter]

it probably was ready to die anyway

I used an 'ol phrase when I did backyard mechanics; "if it don't fit, force it, if it breaks, it was bad to begin with"
Supposed to get rain the next couple days so I can get back to *real* work on the amp, the sun is so addicting
If the tubes don't arc with the SE, I will buy a PP OT, either as a test/spare, or a fix.
Looking at tube data n web-sites, I'm seeing 6.6K 25W for this amp?
Also, studying the schematic, the Verb *source-drive* comes from the speaker, so this amp reverberates the trem then?

[HotBluePlates]

... Looking at tube data n web-sites, I'm seeing 6.6K 25W for this amp? ...

I don't know.

But you have at least one working transformer, so you could apply a low voltage to the secondary and see what you get on the primary to determine the turns ratio (and turns ratio squared is the impedance ratio).

For power handling, you should weigh the lump you have now. Similar power handling will come at similar lump-weight.

If you frequent suppliers of hi-fi transformers, you might find their "25w" OT is much heavier than your "25w" OT. That could mean they expect full power, to lower Hz, at lower distortion. The heavier OT may sound better, or not, in a guitar amp. There are folks who don't feel some replacement OT's give the correct sound in say, a Fender tweed amp, because the core is too big & too clean.

[shooter]

There are folks who don't feel some replacement OT's give the correct sound in say, a Fender tweed amp, because the core is too big & too clean.

well, temp'd in the SE OT, got 1 step further, 2 tubes didn't arc, BUT, after a minute B+ started dropping, so the cap can has been moved from a suggesting to NEEDS replacing.  1 tube still arc'd over, so it's 2 bad tubes.
The main problem now, the customer is MIA!  nothing for the last7 days!, so I'm shelving it for now.

thanks for the help, hopefully everything is ok with the customer.

[HotBluePlates]

... well, temp'd in the SE OT, got 1 step further, 2 tubes didn't arc, BUT, after a minute B+ started dropping, so the cap can has been moved from a suggesting to NEEDS replacing.  ...

Tack-in a known-good cap. See if the problem persists.

If it does, the tube itself is probably gassy, leading to grid current and shifting bias. A gassy tube could shift bias to be cooler or hotter than you'd get with a non-gassy tube.

Or go whole-hog: you already have a couple arcing tubes and possibly one or more gassy tubes. Toss them in the trash for 4 new output tubes. You suspect the bypass cap (part of a can), so just replace it. There are tools to make a 1-minute test of the cap, but it's now a week later. Knock it out & move on (pending the customer returning).

[shooter]

Knock it out & move on (pending the customer returning).

And that's what I'm doing, if it's my money/amp, already done, spending other ppl's money, that's where I *try* and make sure, twice, unlike my UNCLE!

[shooter]

Good news, the owner is still around!  Finally making progress, He has a basement full of old Hammond populated chassis, money does seem to be an issue, so he asked if I could mod this for 6V6, using his parts.  Explained what that would do to *original*, what it would entail, including more bad things as I fix broken one's, along with what still needed to be good new.  IF this was anywhere close to good or better condition, I'd pass, but since it has a wood, spray-painted, label maker, faceplate, rusty chassis, I agreed.

Assuming I get 4 good, old stock, 6V6's with corresponding OT's, I was gonna sorta *copy/paste*, something like a 5E3 PA section, (any other self biased 6V6 PA ideas welcome).  My only concern, the PA tap is at 380vdc, G2 about 360.  I was gonna knock that back, hopefully with an *extra* node prior to the original PA tap, try and dial it around 320/280.  I know the newer tubes run at 400, just don't want to see this back *next week* for blown, old stock tubes.

[Paul1453]

I like the sound I get from my 6V6 Plexi running B+ at 325V.

You don't get the full output power you could,
but the old tubes seem to last forever when they aren't overstressed.   

[shooter]

Brought the bad channel to life, but waaay to much life!  Idle current is 180mA each tube, 51vdc at cathode, 470ohm Rk, 300vdc at plate, 284vdc at G2.  If my math is correct, I’m at 27W a tube. (51/470*249).

The customer gave me 2 AO-29 (M3), and 1 AO-43 hammond chassis as donors for OT’s.  I chose the AO-43 which is 2 6BQ5 PP because it was same physically as the original. My tube charts show like 8K as happy for either 6BQ5 or 6V6, I didn’t look-up the AO-43’s data to be sure.

I replaced the Ecap with Dougs JJ40202020, re-wired the power rail (see schematic), and re-wired the tube sockets for 6V6’s.  The 6V6’s are new out of my last build (PSE).  Everything is about the same as that amp except plate on my PSE is 265vdc, 260G2, Rk 470, Vk about 17vdc. (both amps have individual Rk, and cap)

I’m sorta stumped now, if I burn off another 15vdc plate, my tube chart for 6V6 shows -19 for AB1 PP, which I’m assuming is fixed bias, since it doesn’t say.  I can’t see current dropping that much, I can see it if I burn off 35vdc to where my PSE is, but that’s a lot to burn off, the original plates for the 7591’s were at 365.

Any ideas?  I do have the PT’s from the donors, and the OT’s from the M3’s.  The amp does play, is quite, and while playing average Vk is about 32vdc.

Thanks
dave

[HotBluePlates]

Brought the bad channel to life, but waaay to much life!  Idle current is 180mA each tube, 51vdc at cathode, 470ohm Rk, 300vdc at plate, 284vdc at G2.  If my math is correct, I’m at 27W a tube. (51/470*249).

They'll be dead shortly. The JJ 7591 data sheet specs maximum cathode current at 85mA.

I would suggest getting idle bias under control. You need to unhook any coupling caps going into the output stage, and look for paths that could be short-scircuits or leaking current.

[shooter]

They'll be dead shortly

It's re-wired for 6V6's now, but ya, they to will be dead fast!

unhook any coupling caps going into the output stage

That should happen after work tonight.
Also gonna temp in an OT from one of the Ao-29's.
when I did dcr checks on the OT, they looked good, the tubes operate fine in another amp, power-rail ohms to ground are in the Meg range, volts are stable, pin 8 to ground 470 ohms, pin 3 to OT CT about the same as dcr outta circuit was, but you are correct, something's sucking current like it's free!

[Paul1453]

Your excess current draw problem has to be right near those tubes, IMO.

With cathode bias your resistors should give you around +19V on pin 8 I think.

Instead of -19V on pin 5 with fixed bias, you should have ground on pin 5 and +19V on pin 8 with cathode bias, right?

You have your grid leak resistors connected to ground on the other side of your grid stopper resistors, yes?
I left the grid leak resistor off on a simple circuit on my breadboard.
That did not cause the tube to draw excess current.
On the contrary, the tube started off with normal current draw, but when the grid loaded up with electrons the current dropped dramatically.

So, if your cathode resistors are good and you've got the grid leak resistors hooked up you should have ground on pin 5.
If you don't, I suspect your coupling cap(s) from the PI are leaking voltage onto your grid and screwing up your cathode biasing.   

How does that sound to you?

[shooter]

progress, before I popped out the input caps I traced out the grid circuit, the grid leak R's are ty'd together then run through the trem intensity pot for ground, who-ever replaced the pots before me only put 2 of the 3 wires on that pot!  they left off the ground!!  So I'm confident that's what killed the amp.

I still have a 12khz parasitic coming in on the signal path prior to the PI, but that's with the other side un-tubed.  Hopefully it goes away after I get good tubes in.  I also have a sharp negative transient spike when you dig hard on the pick, but that will fall under, additional repairs

the new #'s are bias 21, plate 330ish, Pdiss 14.8W.  She sounds very good, IF you can ignore that sharp transient in the signal.

[shooter]

parasitic osc is gone, went away when I re-connected the NFB to my bench speaker

both channels tubed and working, but I can still *dial* 52volts DC onto G1 by adjusting the intensity pot, so in the morn I'm gonna replace the .1 cap feeding the pot, trem does osc.

[shooter]

She's runnin good!  replaced the .1 from the trem, still had 1.5vdc on 1 tube, swapped the coupling cap a 2nd time, same, swapped tubes, followed the tube, grabbed a spare donor, 70mV!

the 6V6 rebuild side is running 13.5W per tube, the 7591's 17.2W
very nice tone - both side dials set the same, yields about equal tone-volume.
was a great fix, thanks for the direction and sounding board!