Barn Fresh GA-35RVT came in this afternoon for evaluation(pixs of Fender/Gibson)

[P Batty]

There is a heavy coating of "gunk" on the tanks RCA connectors, did you try to take a reading from a RCA cable plugged into the jack? I'm thinking that a probe tip might not be able to make a connection through all that crud.

[Platefire]

I didn't plug a cable into it but I took readings first off the rca plug, then off the inside rca terminals where the wires connect rca from transformers and also where the wires connect to the transformers. I was looking for a possible bad solder joint in the connections but I get continuity from rca plug to transformer terminals on both input and output transformers. So both hot and ground are getting from the rca jacks to the transformers. I really don't know how to check any further. I did attempt continuity and ohm test from transformer to springs but can't get any ohm reading of continuity there. Like I said before I did get ohm readings from one end of the springs to the other, so the springs are ok and not open at any point. Platefire

Edit: At your request I tried a set of rca cables in it and cleaned the jacks with contact cleaner. All the same except I think it was the input I told you that has 177 Ohms but it's the output that has that reading. Still nothing on the other(input). I still will try the tank when I test the amp upon fire up.   

[Platefire]

In replacing the OT, the old transformer had the primary and secondary wiring going through the same chassis hole. I've always tried to separate these wires and have separate chassis holes for them. Is keeping them separate a waste of time or not? Platefire

[sluckey]

Is keeping them separate a waste of time or not?

yes

[Platefire]

OK, Thanks! I'll just run them all through the same hole as they did before. Except I think I'll find a grommet for the hole. Platefire   

[Platefire]

Installed the new OT, used reverb transformer, powered it up on a light bulb limiter and all seemed good so I fired it up with full voltage. Voltages are running mostly 10 to 20 volts higher than the schematic but I think that's pretty normal for vintage amps on current wall voltages.

Performance is very poor in the the volume in both channels are weak. The normal channel has a little more ohmpth than the reverb but both are still overall very weak. Tried all my normal things cleaning pots, trying different preamp tubes, chop sticking around the circuit---nothing so far has improved the output performance. The power tubes are a used pair of 7591A's I took out of an old Heathkit amp, so I don't know if they are up to par or not. Don't have any replacement to try. It came with a set of 7355's which from what I've read is a poor substitute for 7591's. I was thinking about trying them to see if it would perk the power up, but I read it would work as long a you weren't using pin 4 as a connector lug for something else--well pin 4 has the 1K screen resistor going to pin #8 screen.

One good thing, the tremolo works. Plugged the footswitch in and it does turn it off and on. Haven't tried the reverb yet. I've got to make a cord with a male rca on one end and two connector clips on the other end to connect to reverb transformer output connectors. Then I can plug into the used tank to see if I got anything on that.  

Regarding sound---Awful sounding. The tone is a thin trebbly mess. Worst guitar amp sound I ever heard.
Too much trouble to revive an amp like this just to get a sound like this. I'm hoping something is wrong and when it is corrected it will be much better? Platefire  

[sluckey]

Regarding sound---Awful sounding. The tone is a thin trebbly mess. Worst guitar amp sound I ever heard.
Too much trouble to revive an amp like this just to get a sound like this. I'm hoping something is wrong and when it is corrected it will be much better?

I bet it's still sick. Try tubes first. If that doesn't perk it up then measure voltages and go from there.

[Platefire]

I did try the 7355 power tubes that came in it. The power perked up a little but the normal chan was still hotter than the reverb chan. The plate and screen voltages dropped pretty drasticly from screens 425 to 385 and plates from 439 to 390 on one and 420 to 374 on the other. I think I need a new matched set of 7591 power tubes. Platefire

[P Batty]

After you try the new tubes, check out the third video on the Lancer:

https://www.youtube.com/watch?v=p2yFgikXrLo

I think it might be the answer to your tone problem.

[Platefire]

P. Batty

Wow! That is a terrific well made video with awesome results. I've been looking at those resistor/cap networks and wondering about them myself because there is nothing like that in most other amps. That's very encouraging! Thank you! You've been such a help on this project. I will have to confer with the customer before making those circuit changes, but I think all I'll have to do is let him hear the amp as is and watch the video and be will be a believer.

On an added note. I thought the tremolo wasn't working at first after I fired it up because turning the control knobs had no effect. I hooked up the footswitch and it started working. So it has to have the footswitch plugged in to work. So without the footswitch I would have had to jumper the footswitch connections to turn the tremolo on and off by turning knobs only--which in a live playing situation is not convenient. As one who uses tremolo/vibroto myself in certain parts of a song quite a bit, the switching ability is a must. So the footswich you sent is working out great. Platefire

EDIT: Had to add this that I tried after the above post. On the reverb channel I put a jumper around the resistor/cap network after the volume pot to V4b and it's like the channel came to life! All the tinny metalic tone was gone and the volume picked way up. All I can say is "What a difference"! If anyone would like to look at this, the schematic is attached. Why would someone in there right mind put such a
performance blocker in a circuit is beyond me?

[Platefire]

If you could take time to watch the video in reply 60 regarding mods to this amp for improving the tone, I have questions????

A-Normal Channel. There are two resistor/cap networks on this channel to be removed. First one after 250K Loudness/volume control---he said to remove the 330pf, two 220K resistors and .0047 cap to ground. He said replace this network with a .02 cap and install a 1 meg from grid to ground prior to V4a.

My question is--is the .02 cap really needed? you have a .0047 cap after V1a gain stage and there is no other DCV in the circuit that I can see all the way up to the grid of V4b. So it appears to me just a wire from vol pot output term to V4a grid and a 1meg grid to ground would do. Am I right or wrong?

B-Reverb Channel. Same thing, no DCV into this area either that I can see. Not sure the 68K in line grid resistor is needed either. Also would a 1meg to ground be needed here too? Am I right or wrong?

A schematic is attached to reply 61 if you need to look at a schematic but he references the schematic on the video and it's pretty clear on there. I just don't want to put more parts than needed. Thanks, Platefire

[PRR]

> the 68K in line grid resistor

I think that is the mixing resistor where the reverb return comes in.

A hasty bypass with little fault is to tack large caps across the 300p caps. Like 0.1u. Another is to lift the end of the to-ground cap.

[sluckey]

My question is--is the .02 cap really needed? you have a .0047 cap after V1a gain stage and there is no other DCV in the circuit that I can see all the way up to the grid of V4b. So it appears to me just a wire from vol pot output term to V4a grid and a 1meg grid to ground would do. Am I right or wrong?

You don't need the .02 cap. And you don't need the 1M resistor either. Just remove that cludge RC network and run a wire straight from the vol pot wiper to the grid of V4b.

B-Reverb Channel. Same thing, no DCV into this area either that I can see. Not sure the 68K in line grid resistor is needed either. Also would a 1meg to ground be needed here too? Am I right or wrong?

Just remove that cludge RC network and run a wire straight from the vol pot wiper to the left end of the 68K resistor. You don't need to add a 1M resistor, but you need to leave the 68K in the circuit.

The third RC network is common to both channels. Just remove that cludge RC network and run a wire straight from the junction of C9 and R15 to V5A pin 2. No need for a 1M resistor here either.

[Platefire]

Thanks Sluckey! I was pretty sure about not needing the extra .02 caps but wasn't for sure on the 1 meg to ground resistors. As you and PRR said, the 68K needs to stay. OK! Platefire

[Platefire]

OK, I jumpered over the reverb RC network and also the RC network common to both channels with alligator clip patch cords just to get a feel of what the modded results would be. Tested with two extreme cases, a tele and a Les Paul. The response, overall tone and volume is so much better but to get a tone that I really liked I wound up with a tone setting with the treble on 10 and bass on 0 for both tele and paul. The paul needed a treble boost even with that. This is too extreme of settings and you shouldn't have to go that far to get close to a good setting. I guess the tone stacks could be rewired for Fender T/B type but then the pots would probably have to be changed out. Maybe just a 120pf bright switch accross both volume pots would hedge up the difference. Also I might ought to put a new set of power tubes in to see what that does before I make any decisions about tone stack re-working or bright switches. I am presently running a used substituted set of 7355's for 7591's and don't know what shape they are in other than they are working. The RC networks need to come out for sure.
     Also after I jumped the RC networks the vibroto started acting up. Instead of ticking it was making a sound more like "whomp whomp whomp whomp"--any idea whats going on there? Platefire  

[Platefire]

Two things;

Vibroto has repedative whooshing sound when the depth is turned up past 12:00 oclock. This only happens when the RC networks are bypassed--works fine without the bypass jumpers. Something in the vibroto circuit must need adjusting to make up for the removal of RC networks. I've looked at the schmatic trying to figure out if the caps or resistors on the depth is what needs to be changed? Or does that R53 board mounted 500ohm pot on V5b need to be adjusted?

Tried the reverb. Didn't work. However you can shake the tank and get spring noise through the amp. So as my previous Ohm test indicated, nothing on the input and 177 ohms on the output on tank. I doubled checked continuity through the cables and that was good---so no shorts in the rca cables. Both reverb transformer output leads read 4.5 ACV. When I click the reverb on with footswitch a  slight hummm appears. What I'm wondering if I hook an un-matched Ohm reverb tank from another amp to this amp--could it harm the tank? I thought if I got some kind of reverb to the amp even though un-matched ohm input/output that would let me know for sure it's the tank needs replacement and the reverb transformer is doing it's job.

Trying to get all this settled so when I call the customer, I can give him a full report regarding everything. Platefire  

[sluckey]

Vibroto has repedative whooshing sound when the depth is turned up past 12:00 oclock. ...does that R53 board mounted 500ohm pot on V5b need to be adjusted?

Does the tremolo sound OK when the depth is set to a lower level? Adjusting the pot on V5B will change how brightly the lamp inside Z1 flashes, which will affect the strength of the tremolo. Turn the amp off and connect an ohm meter between ground and V5 pin 8. Write down the resistance reading in case you need to set the pot back to where it is now. Now turn the pot either max CW or max CCW. Leave it at the max end that gives you 500Ω, NOT THE END THAT GIVES YOU ZERO OHMS. That will make the tremolo weaker. Now turn on the amp and turn the DEPTH knob all the way up. Slowly adjust the pot on the board for a pleasing maximum strength/depth tremolo sound.

Both reverb transformer output leads read 4.5 ACV.

What does that mean? Just check the resistance of the RT secondary with the tank disconnected. Should be very low. The 4AB3C1B tank like used in all the Fender tube amps should work.

[Platefire]

Sluckey

The vibroto depth sounds OK with no whooshing at lower setting but is too weak to be usable. When you turn it up to the usable area it starts whooshing. I did an Ohm check as you said and it's set at 256.8 Ohms.
I haven't tried adjusting it yet. Got to go somewhere. When I get back I'll put the jumpers over the RC networks again and try adjusting it like you said to see if that helps it.

On the 4.5 VAC on the RT, I just measured the AC on the secondaries with the amp running because I knew the RT is like a capacitor and lets only low AC pass--didn't know if that reading would tell us anything or not. On reading the RT secondaries with the amp powered down, I didn't know if you wanted me read Ohms from the plate wire primary to the secondaries or read accross the secondaries so I did both. No Ohm reading at all on any range setting from primaries to secondaries. Reading accross the secondary terminals I get 426 Ohms. So what's the verdict on the RT?

Thanks for your Help!!! Platefire

BTW--hate to sound like a total dummy but how do work the quote tool---tried and couldn't get it to work?

[sluckey]

Go back to youtube and watch all three of the videos about the renovation of the GA35RVT. Good info about the reverb tank in the second video. Pay attention to his reverb problem and his solution. Just may be your problem too.

Reading accross the secondary terminals I get 426 Ohms. So what's the verdict on the RT?

That should be OK. The tank is a Gibbs Model C which is the same as an Accutronics 4FB2A1C according to the guy on youtube (I didn't verify). The tank input impedance is 1475Ω and the output impedance is 2250Ω. The 426Ω DCR of the RT secondary would be in line with that high tank input impedance. A typical Fender tank will not work in this amp.

how do work the quote tool

I just click on the 'Insert Quote' icon. Then I copy/paste the text I want quoted between the quote tags.

[Platefire]

Sluckey

I went back and watched the videos. On his reverb transformer the secondary output terminals were loose and coming out on the video. My RT the terminals are tight. I'm not too anxious about tearing into it when the terminal appear to be in good shape and tight. From what you said, I assume hooking the reverb tank from my Allen Accomplice(DR AB763) wouldn't work for testing purposes to see if the RT is working. I would like to tell the customer that a new reverb tank would fix the reverb--but I need some kind of test to determine that for sure! It would be bad to have him pay for a new reverb tank installation and still not have anything.

On the vibroto I went back set the pot CCW to full resistance. Put the jumper clips over both RC networks related to reverb channel. Turned the amp on with the Vib level full up and turned the adjustment pot CW until I started getting some good lively vibroto action. I adjusted it back and forth a couple of time to try to zero on the sweet spot. I heard no more whooshing this time. At the present time it's working good with no side noises.

Also with the RC networks out the channel needs some more brightness in it. I got out some more alligator clip jumpers and tested a 47pf, 100pf, 390pf and 500pf Silver Micas between the volume pot input and output terms. I liked the 500pf best. It brings some brightness back into the channel without the tinney metalic sound it had before. I'm thinking the 500pf may would work as a permanent fixture, not even switchable. That's a serious consideration!

The thing is starting to sound really good to my ears. I'm thinking if Gibson had of done some of this tweaking way back when, Gibson amps may would have a lot better reputation today. Platefire  

[sluckey]

There's no need to tear into your reverb transformer. Your problem is different than the youtube guy's problem.

I would like to tell the customer that a new reverb tank would fix the reverb--but I need some kind of test to determine that for sure! It would be bad to have him pay for a new reverb tank installation and still not have anything.

You've already proved the tank is bad. Back in reply #53 you said "Edit: At your request I tried a set of rca cables in it and cleaned the jacks with contact cleaner. All the same except I think it was the input I told you that has 177 Ohms but it's the output that has that reading. Still nothing on the other(input). I still will try the tank when I test the amp upon fire up. ". If you trust your resistance readings then the input transducer coil is open. A tank with an input impedance of 1475Ω will read a dc resistance of approx 200Ω across the input jack.

AES has two 4FB tanks for about $18. Both have the input/output impedance that you need. Do you have to ask for permission on every penny you spend to fix this amp? The customer has to expect to spend some money. See this link...

      http://www.tubesandmore.com/search/node/4fb

And here's some good info on reverb tanks. Save this file...

[Platefire]

Thanks sluckey for the info and being patent with me. Regarding money spent on this project, I told the customer that the first major step was to get the new OT in and get it up and running and then I would know what we got and I would get back to him at that time. He hasn't told me to fully restore everything. So I'm just trying to assemble an accurate picture and approximate cost before I call. I've pretty much researched and assembled all that info with the reverb being the last thing I was unsettled on but now I think it's clear now, most likely a new tank will put the reverb back in operation. I guess honestly I'm doing all this extra because I love a project like this and it is for a personal friend and fellow musician that go way back.  

On the Gibson Mod tanks you referenced, I had looked at those at AES and wondered if they would be OK since they had the right input/output impedance. I guess I've had a wrong assumption thinking the Ohm measurement on the input/output of the tank should match the ohms it was rated for--but you say it should be in the 200 Ohm range--I didn't realize that! Also looking at the spec sheet you attached with my tank # being 4FB2A1C, I'm a little confused about the mounting which was "C" vertical mounting connectors up---seems it should be "B" Horizontal connectors down(like fender). Not sure that's a concern but in the spec sheet it did say the direction of pull of gravity was considered in the design. I'm wondering from that designation that maybe the tank was originally mounted vertically on the cabinet bottom back plate?

So on the AES Mod tanks it looks like the AFB3A1B--long delay(3), both input/output grounded(A) and horizontal mount connectors down(B) would be correct for mounting in bottom of cab. Only variation is original was a "2" medium delay. I found an exact match at Studio Sound Electronics, Revisit Reverb Tanks (link attached-scroll down to 4FB2A1C) but the cost is $34 plus shipping=$42 but AES price is a lot more reasonable. Platefire

http://www.amprepairparts.com/reverb.htm#4  

[sluckey]

The tank you need is the 4FB3A1B from AES. The schematic says you need grounded connectors and the fact that the tank will sit horizontally in the bottom of the cabinet with the open side down means the last alpha character should be "B".

[PRR]

> thinking the Ohm measurement on the input/output of the tank should match the ohms it was rated for

I may have missed your numbers. But the *audio* impedance is significantly higher than the DC impedance (what you see with an ohm meter). IIRC it is like 5 or 10 to 1 different. So a "8 ohm" tank input will measure like 1 Ohm on the meter. There are better numbers in Steve's paper.

This is no different than an OT. A "4K" OT measures like 100-300 Ohms with DC.

It is similar-but-different from a loudspeaker. A loudspeaker's losses are so great that an "8 Ohm" speaker will be 6 or 7 Ohms at DC. It doesn't rise so much for AC/Audio. (Except at 80Hz and above 4KHz.)

A side-point: if you are going to fix customer's amps, it does not hurt to have a spare reverb tank in the shop. It's a fairly common fault. And as you know, not easy to *prove* the tank is at fault unless you can swap it. So it's a shop-supply.

[Platefire]

The customer came by today. I let him hear the amp as is and then with the RC networks jumpered and he agreed that is was much better with the RC networks bypassed. He said take-um out. I've already removed all three RC networks and it sounds even better than with the jumpers. The jumpers were picking up a lot of noise--much quieter and tighter soldered up. It now sounds a little brighter to me also, so I didn't put the 500pf bright caps in there. They can always be added latter if needed. He told me to go ahead and get a new set of matched 7195's, reverb tank and speaker.

After I first installed the new OT and fired it up for the first time, it sounded so bad and volume was so weak.
Thanks P Batty for pointing out the #3 restoration video on Improving tone. I'm still amazed at how much these 3 RC networks in the grid path was killing the sound and output of this amp. What were they thinking??  Platefire
 

[Platefire]

I may just go ahead and install adjustable fixed bias as shown on this video. I've presently got fixed bias but not adjustable. Only problem in the space where he shows his mounted, a previous mod to my amp installed an extra board in that space mounting the filter caps on the new board--which is a good thing but I'll have to find another space to put my 10k pot. Attached is a picture of my same space. I think either side of the terminal strips, mount pot through chassis where adjustment would be outside--and if I do that, may as well install external check points with 1ohm resistors.

Converting a Fixed Bias Amp into an Adjustable Bias Amp

[Platefire]

Got to noticing after the RC networks were removed from both channels that the normal channel was brighter than the reverb channel. I thought other than one having rev/trem and the RC networks they were the same! I took a closer look and found that the normal channel has a .0047 after the first gain stage where the rev chan had a .02 cap after first gain stage plus a 68K resistor just before the second gain stage. So now the normal channel is more like a bright channel and rev chan like normal except with rev/vib. Platefire

[Platefire]

Got the reverb tank in and the new set of matched 7591's.

Installed the new tubes and fired it up and it sounded awful!!! I was expecting sonic blast off! So I got to checking what the problem was and found the amp had been modded to adjust the fixed bias circuit for the substituted 7355's tubes. In the bias circuit they changed R41 from 7.5K to 5.5K and R40 from 47K to 72K. That's why the bias voltage is reading -31 instead of -20 on the schematic. Not sure if I want to just put the old resistors values back in for the new tubes or mod it to fixed adjustable bias with 1 Ohm cathode resistors and bias test points?!

Also hooked up the reverb and the reverb was working but I was getting this terrible hum. I went over everything trying to figure it out until I finally gave up. Came back latter to tackle it again and started trying some other stuff and just happened to push the tank away from the PT and the hum diminished. What I had done was set the tank up on the bottom of my wooden "U" chassis rack--and that was the problem. When I moved the tank out of the chassis rack on the table it all went away. So then I could focus on the sound of the reverb. It sounds OK but I have to turn the pot past 50% before the reverb starts showing up. I ended up with the reverb set at about 75% of 3:00 oclock postition to get the amount I find at 2 1/2 of 3 on other amps. I don't know if this is a circuit problem or a tank mismatch issue? The input and output Ohms were matched to what they were suppose to be. Maybe I should change theV2 12AU7 driver to a 12AX7!? Platefire

[Platefire]

Considering modding this amp for fixed bias to adjustable fixed bias with test points. Only problem on the externally adjusted bias pot to drill a hole through chassis. I have very limited space around the bias circuit to install pot to chassis adjacent to bias circuit---one within about 1" from PT, the other about 1" from power cord entrance through chassis. Only other options is make some long leads to make some distance. The bias circuit is actually located right under the PT separated only by chassis--so it don't seem like having the adjustment pot close by also would cause a noise problem--if it hasn't already. Will it? Platefire 

[sluckey]

You could use one of Doug's tiny bias pots and mount it on that terminal strip. I see some spare lugs.

[Platefire]

Thanks Sluckey. That's such a nice illustration, but I was talking about how to mount a pot through chassis to allow external adjustment to go along with banana jack test points. I have Doug's 10KL bias pot with the shaft/nut and internal slot adjuster. Are you saying, in this circumstance it would be better to just use an internal adjustment?

I'm really thinking now just changing the two resistors that the previous tech installed to re-bias for the 7355 tubes, installing the original schematic values 7.5K & 47K back in there and see if the new JJ 7591's are happy with that. Only problem I don't have a 7.5k. I would have to put a 4.7K and a 2.7K in series that measures 7.41K. I haven't discussed with the customer about modding it to adjustable bias with external test points that would include some hole drilling. Don't know if he would want that or not--probably would if I recomended it. So it wouldn't hurt just to put it back stock and see how it does and if it isn't happy, then discuss adjustable bias mod. Platefire    

[sluckey]

Are you saying, in this circumstance it would be better to just use an internal adjustment?

No. I was just offering an alternative that will easily fit and be very easy to install.

[Platefire]

Well I've done several things  

Changed the bias circuit back to stock and it seems to be happy, sounds good and no red plating with the new matched set of tubes.

Built reverb cables with regular shielded amp cable/90 degree male rca's, build reverb bag and now tank is resting happily in bottom of cab. I installed a 12AX7 in V2 reverb driver in place of the stock 12AU7--so far I think it sounds better and stronger.

Installed a regular fender cab handle to replace the old disintegrated Gibson handle.

I installed the upper back plate upside down on purpose as with that panel right side up it blocked the view of all the tubes plus blocked ventilation trapping heat under chassis. I think you should always be able to view your power-tubes to check for red plating.

The only weak link now is the speaker. Customer didn't say to find a replacement so I didn't. Platefire  

[P Batty]

That was quite a saga, I've been tempted to buy another mid 60's Gibson Saturn (I bought one new in 1965 and sold it in 1967) but your heroic efforts have probably changed my mind for good. Still, it's good to see that you were able to make a good amp out of a discard.

[Platefire]

Well Thanks! And Thanks to you for the very much needed parts. The reverb or Vibroto will not work unless the footswitch is used. The only other way around it without a footswitch would be to hot wire them on all the time, and nobody wants a vibroto on all the time. So the footswitch and reverb tranny was intrumental in making it work right. That footswitch was very different than normal 1/4" plug type and it was just awesome to get it. I also used the wood & cardbard base off the old reverb tank on the new tank. One of the treaded chassis mount nut/inserts was missing plus two chassis mounting screws were missing--so those you sent helped correct all that. So you have a big hand in this to!  

Taking those resistor/capacitor networks out of the preamp circuit did wonders for opening the amp up with overall loudness and tonewise. Again--you pointed me to the video about removing the RC networks. I've got a chance to get a Emenance Cannibas REX in like new condition for $40.00 tomorrow-so if all goes well I may have a speaker for it. To be honest I'm burned out on working on this amp and really ready to get it complete and move on. I really don't want to send it out the door with that no good speaker in it. I'd feel much better if it was purring like a kitten. Platefire  

[Platefire]

Well I picked up the Emenance Cannabis Rex today. I wouldn't fit! The OT has to fit right up in one of the openings in the speaker frame and these openings was smaller and no matter how you rotated it, it wouldn't line up correctly. Boy Gibson pulled one when they did this! The only true correction for this problem is to build a new speaker baffle and locate the speaker hole away from the OT. Platefire

[Platefire]

On this one the customer has had it back for about three weeks now. I heard from him today and he's wanting me to work on a couple of his guitars. Got to asking him about the Lancer and he said he's loving it. He had an old Fender SS Princeton Amp with 2-10 speakers that didn't work anymore. He built a new baffle for 2-10 speakers to fit the Lancer and installed the the 2-10's out of the old fender in the lancer. I don't know how he's matched up for Ohms regarding the OT now but he's happy with it. Platefire

[P Batty]

The mismatch shouldn't be too bad, 4Ω instead of 8Ω. Or is it 16Ω? He probably will never blow up those speakers. At any rate, the bottom line is that the customer is satisfied!

[Platefire]

This amp just came back in with the customer saying it was loosing power and make a crackling sound. I turned it on and after a few minuets one tube started red plating. Platefire 

[DummyLoad]

may fit?

http://www.classictone.net/40-18088.pdf

[DummyLoad]

This amp just came back in with the customer saying it was loosing power and make a crackling sound. I turned it on and after a few minuets one tube started red plating. Platefire

i had a CMI SG systems bass amp that used 8417 tubes do that. B+ was in spec, bias in spec, replaced sockets and coupling caps and grid leak resistors. was a bad OT. apparently had some shorted windings on 1/2 primary. first time i ever replaced an output transformer that wasn't outright fried. it took a while for the tube to start red-plating - IIRC, a few minutes after warm-up.


--pete

[Platefire]

Well I haven't pulled the chassis yet. I thought I had another used set of 7591 tubes to try but couldn't find them. I wanted to see if they red plate also. I guess I can just switch the tubes and see if the red plating follows the tube or stays in the same socket. I installed a new Classic Tone OT when I worked on in February 14. Platefire

[Platefire]

Just did some more testing. Fired up the amp. It came on normally with good full power. Noticed the reverb wasn't on so I turned it on with the foot-switch and it came on but with a loud hum. I unplugged the foot-switch and the hum stopped and the reverb was then working nice and quiet. So I began playing my guitar for a couple of minuets, it was really sound good and then I heard a little "pop" and sound dropped to nothing just like the flip of a switch. Also I noticed the left 7195 tube started red plating. I then turned it off.

I then let it cool off a few minuets and switched the power tubes to the opposite sockets.  I then turned it back on. Again when it had normal operation for a minuet or two and then cut off like a switch again. The red plating quickly followed and it also followed the power tube swap to the right socket. I then turned it off.

Don't know if a power tube would act like that. Never had one just work normally and cut off like that. I thought I had it biased correctly when I rebuilt it. JJ tubes usually take a lot of abuse and keep on ticking from my past experience. I could see a bad coupling cap, or bad bias or bad tube red plating but the instant cut off has got me puzzled. I'm wondering if it could be some kind of short? Next thing to do is pull the chassis and start doing checks!

Any wild guesses what it may be? Platefire   

[sluckey]

The red plating quickly followed and it also followed the power tube swap to the right socket.

Doesn't that make you suspect the tube???

[Platefire]

Yes, simply because the red plating followed the tube. I just never had a tube that would work good for a minute or two and then cut off like a switch. Also I can't help but wonder if a fault in the circuit killed the tube Prematurely? Platefire

BTW--Another thing that I noticed when it went into cut off in both the tubes---a blue glow around the inside of the glass that wasn't there before cut off. If one tube was shorting out, could it be doing all that?

[sluckey]

I would check all voltages around the 7195 sockets. If they look normal I would buy a NEW set of tubes to put in it.

[Platefire]

Well being a little lazy I used my test adapter in one 7591 sockets and used the test tabs with my MM. This is with the tubes installed under a load.

1-Plate voltage pin 3=408V, schematic says 410V

2-Screen voltage pins 4,8=413V, schematic says 409

3-Bias voltage pin 6=-17.88, schematic says -20.5V

My 7591 Sylvania data sheet says for AB1 operation grid 1 bias should be -16V at 400V plate voltage and -21V at 450V

Really need to install 1 Ohm resistors to get Mv/Ma. According to the Weber bias calculator an AB PP 7591 pair at 408V plate voltage should be biased at 25.7 Ma. Platefire

[Platefire]

Got a new set of JJ 7591's ordered for this one plus I'm going to install the external test points with 1 ohm resistors. I've got one of those 10K bias pots that goes through the chassis that can be adjusted inside the amp or outside with a screwdriver.
The most convenient place to mount the pot is right adjacent to the PT--will that work? Thinking about trying it anyway just to see. Next most convenient place is remove the extra external speaker jack and put it there. platefire

[Platefire]

Had to show some pixs of what the customer did to this Gibson Lancer. He robbed his old SS Princeton Chorus of the speakers(2-10) and name tags and stuck it on the Gibson. Now its a Gibson/Fender. I had it biased to hot and burned up the original tubes. Got a new set of JJ 7591's, new 1 Ohm resistors with external test points and 10K Bias pot through chassis by PT. Got it running a bit cooler now. Nothing like destroying a good old vintage amp. I figure my bias test points & bias pot won't make much difference after all his changes.
   If you would like to hear a song by Luke(customer) recorded on this amp please follow link.  Platefire 

http://www.reverbnation.com/thefaithfulgiantkillers/song/22205939-covered-by-the-blood

[Platefire]

Bump! Ya'll need to see this so I bumped it up.