Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on April 24, 2023, 06:28:21 pm
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On my SVT setting the bias has no problem to set to .072 like the instructions state
With the balance ,the pot is not doing much to set to .01. The whole swing of the pot is .000
Plus it has a hum so it may be time for caps
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Open it up. Check the screen resistors . One of them may be blown open
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Thanks, I just bought this off a friend. It was a very good deal but I suppose it needs work. Since I'm not that familiar , I not sure if the hum that I hear is normal or that it needs caps. I thinking they need them because he told me that they were done around 2000.
The balance and hum pots are very noisy and need a cleaning.
I've watching some SVT repair videos and it seems that there's quite a few things to look for. One thing are the yellow box caps , if it has them. There's a history of 2 resistor's that go in the pre-amp.
On thing is that I tested the power tube heat and each one was around 90c. Not sure if that's common test but at least you know if a tube is running out of control.
The amp is still together. I was able to get my hand in there just to tap the pre-amp tubes . There's a a noisy JJ ECC 832, for channel 2 so I had a 12ax7 to swap that out. That one makes channel 2 very scary. I may go with a lower gain. The 6C4 was very noisy so I'd have to find one. A super light tap makes that go.
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Nope. First thing to check in those is the screen grid resistors. If one is open, it will throw out the output transformer current, resulting in hum. When all tubes are conducting, it’s easy to get the bias comparator working properly. So check those Rg2s with your R-meter. Which means taking the boards out. Sorry, it’s the only way to do it on a SVT-CL
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If the tubes are very badly balanced / matched with too much difference they can generate hum.
Just measure the cathode current of each tube and you will know.
The filter caps; You have to measure the AC component on each of them. More than 0.3 volts, it is better to replace them. Otherwise they are not the cause of the hum.
Or if they show signs of damage; bubbles at the ends for example.
Wich SVT ?
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1975
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This one ;
removed incorrect schematic... sluckey
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This one ;
Thanks, which one is the pre-amp page?
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Look here ; https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_Schematics.htm
I think it is this one;
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Look here ; https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_Schematics.htm (https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_Schematics.htm)
I think it is this one;
Thanks!
Can you suggest where to go get the cap kit? I know that Fliptops carries it but I'm wondering where else I could go?
https://www.fliptops.net/catalog/p-100084/cap-kit-for-ampeg-svt-w6550-v.1
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Open it up. Check the screen resistors . One of them may be blown open
So you're saying all the 47k's off pin 5 to check?
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So you're saying all the 47k's off pin 5 to check?
No. Screen grid is pin 3. Screen resistor is 22Ω.
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yes yes, sorry had grid on my mind too much, thanks.
From what I see on the schematic there's only 1- can cap, but inside there are 2. Plus they are the same value 70/40/40. Looks like the only one available is a CE. Are there any ones better out there? I don't see JJ or F&T making ones that size.
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Also, If I decide to update the grid resistors there's no wattage requirement unless I go 1/2 watt.
What rating would you all suggest?
Any suggestions for a 6C4 tube?
Thanks
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Better verify you are looking at the correct schematic. What kind of power tubes are in your amp?
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Better verify you are looking at the correct schematic. What kind of power tubes are in your amp?
Valve Art- 6550. the tubes have a collar style lock at the base , not a spring and top plate system.
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Valve Art- 6550. the tubes have a collar style lock at the base , not a spring and top plate system.
Well then, the schematic posted by Latole is not the one you need. Look through Hoffman's schematic and find the correct schematic.
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Valve Art- 6550. the tubes have a collar style lock at the base , not a spring and top plate system.
Well then, the schematic posted by Latole is not the one you need. Look through Hoffman's schematic and find the correct schematic.
I think it's this one. It has the 7-/40/40 can cap shown
removed incorrect schematic... sluckey
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Nope. Wrong power tubes.
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Nope. Wrong power tubes.
you're right, maybe the earlier version.I'll check around
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Look here ; https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_Schematics.htm (https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_Schematics.htm)
I think it is this one;
Thanks!
Can you suggest where to go get the cap kit? I know that Fliptops carries it but I'm wondering where else I could go?
https://www.fliptops.net/catalog/p-100084/cap-kit-for-ampeg-svt-w6550-v.1
I never buy a capacitor kit, it's too rare and there are too few good ones.
You will have to buy them by the unit, choosing according to the MFD and voltage values and taking into account the physical dimensions.
You will have to take the values on the parts actually in your amp, not trust the schematic.
Where is your location; North America or Europe to give you the right references
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Boston
I found most if not all from Antique supply in AZ. Very easy site . The carry the CE multi cap, F&T & Sprague
The only odd one was the 30uf/600v. An odd item , but I found a 30/500v.
Any suggestions on the 6c4 tube?
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I know very well Antiques
You are looking to buy a 6C4 tube ?
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I found this one that has 6550's.
I suppose the the cap section outlined with dashes are the can caps?
removed incorrect schematic... sluckey
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I know very well Antiques
You are looking to buy a 6C4 tube ?
Yes, it's too sensitive and noisy even touched
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In reply #2 you mention 6V4. In reply #13 you mention 6C4. Which is it? They're totally different tubes.
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In reply #2 you mention 6V4. In reply #13 you mention 6C4. Which is it? They're totally different tubes.
Sorry for making people crazy-
6C4
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I found this one that has 6550's.
I suppose the the cap section outlined with dashes are the can caps?
That's much closer. However, the two cap cans are different values. You mentioned earlier that the two cans in your amp are the same?
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I found this one that has 6550's.
I suppose the the cap section outlined with dashes are the can caps?
That's much closer. However, the two cap cans are different values. You mentioned earlier that the two cans in your amp are the same?
Well I'm basing my caps on the Fliptops parts list. I'll go by what Latole said and open it and make a real parts list.
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From moving the 6c4 in and out of the socket it may be making a better connection and it's not that noisy. One thing, it's sensitive to tapping like the most v1's are. Besides that it's quiet. Is this normal for that position and tube?
Couple more questions-
In my search for a 30uf / 600v, the most I can find is a 30/500v. Will that be ok or should I try for a higher value at 500v? or what combo would be safe?
For those who've done an SVT are there any tell tale things to look for and change with out testing?
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" One thing, it's sensitive to tapping like the most v1's are. Besides that it's quiet. Is this normal for that position and tube? "
Yes, all V1, any tube, are noisy when tapping
Filter caps ; two 50 mfd 300 volts ( or more ) in series = 25 mfd 600 volts
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The 6C4 is v4, would that matter? I'm saying by being tapped it reacts like a v1 would
Thanks for the cap info!
Also, for one of the can caps, I found a 80/40/40 for the 70/40/40 stock size. Should I go with the 80uf or stay with the stock value?
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The 6C4 is v4, would that matter? I'm saying by being tapped it reacts like a v1 would
Thanks for the cap info!
Also, for one of the can caps, I found a 80/40/40 for the 70/40/40 stock size. Should I go with the 80uf or stay with the stock value?
6C4 ; sorry for my mistake. I don't know .
Read here ; "....that they're prone to microphonics in general and their use in the SVT doesn't help that. "
https://www.talkbass.com/threads/ampeg-svt-6c4wa-tube.1315801/
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Maybe I should get one
Thanks
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" One thing, it's sensitive to tapping like the most v1's are. Besides that it's quiet. Is this normal for that position and tube? "
Yes, all V1, any tube, are noisy when tapping
Filter caps ; two 50 mfd 300 volts ( or more ) in series = 25 mfd 600 volts
I found 2 MoD caps at 70uf/350. Is that over kill? 35/700v
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" One thing, it's sensitive to tapping like the most v1's are. Besides that it's quiet. Is this normal for that position and tube? "
Yes, all V1, any tube, are noisy when tapping
Filter caps ; two 50 mfd 300 volts ( or more ) in series = 25 mfd 600 volts
I found 2 MoD caps at 70uf/350. Is that over kill? 35/700v
Good choice these MOD
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First report-
This amp has never had a cap job. kind of sad to see and I hope my friend didn't pay for one on the service calls he had done.
Pre-amp-
Should the yellow box caps be replaced?
Change the yellow box caps and silver 40/450v caps?
Power amp-
This amp has 2- 100/40@450v volt caps. Not the 70/40/40 that i expected
100@ 450v
90 @200V
30@600V
Should I cut out the .047 white death cap?
Tube sockets-
The 47K's measure from 38-40k. Even the larger one on the end tube socket
22 ohm screen resistors-
5 measure- avg of 22.9
1- won't measure
Change them all or only the bad one?
Should the yellow box caps be replaced?
Are here any other recommended parts to check?
Thanks
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more pics
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More tests-
5 ohm @ 5 w- power resistors-r's-27,30,32,38,41,44
5 measure- around 6
1- won't measure- should I replace only the bad one or all?
For r's- 35 & 36- 1 ohm @ 5w- They're at 2 ohm and not 1 ohm.- replace?
Thanks
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power amp
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More tests-
5 ohm @ 5 w- power resistors-r's-27,30,32,38,41,44
5 measure- around 6
1- won't measure- should I replace only the bad one or all?
For r's- 35 & 36- 1 ohm @ 5w- They're at 2 ohm and not 1 ohm.- replace?
Thanks
These are all goods, no need new one
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Thanks but here's a few questions that may have slipped by-
1- Should I cut out the .047 white death cap?
2- 22 ohm screen resistors- 5 measure- avg of 22.91
1 won't measure.- replace all or only the bad one
possible tube subs-
Original value - 90uf@200v - I have an JJ- 80uf @500- use the 80 or get the 90?
The 47K resistors off pin 5 measure from 38-40k. Even the larger one on the end tube socket- I bought new ones anyway
Thanks
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I think you are brave and hardworking to start repairing this amp with so little knowledge of electronics :thumbsup:
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Thanks, I have an extensive background in car audio. So I've soldered a lot and have designed and installed some amazing systems. Many with 5.1 processors from Jim Fosgate the father of more current Dolby pro-logic.
Plus, with the help of the forum and some other techs, I'm gone through my Traynor YBA-3, Sunn 2000s & converted a Fender '75 Super twin into a bass amp. My first help on that was a tech from NJ Adam who used to work with Dennis Kager, from Central Music. Adam is an SVT expert. Dennis was the service manger at Ampeg/Magnavox before opening Central Jersey Music.
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1- Should I cut out the .047 white death cap?
No need since you have a properly wired three prong power cord. It's not a death cap. It's simply a line filter connected between neutral and ground.
2- 22 ohm screen resistors- 5 measure- avg of 22.91
1 won't measure.- replace all or only the bad one
I would replace only the one that won't measure.
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Original value - 90uf@200v - I have an JJ- 80uf @500- use the 80 or get the 90?
I have a xcion 100@450v. Ok to use that or get a better one?
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Original value - 90uf@200v - I have an JJ- 80uf @500- use the 80 or get the 90?
I have a xcion 100@450v. Ok to use that or get a better one?
Both will work. JJ or Xicon
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a question on security-
I sent a picture of the clamps that hole 2 caps. My question is if can use a mounting tie -
https://www.homedepot.com/p/Commercial-Electric-8-in-UV-Mounting-Cable-Tie-Black-10-Pack-GTM-200STB-10/203531941
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Yes.
I also use steel strapping;
https://www.homedepot.com/p/Oatey-3-4-in-x-10-ft-28-Gauge-Galvanized-Pipe-Hanger-Strap-33526/301505504
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Yes.
thanks!,
In the pre-amp there's a 40uf@450V electrolytic want to change. I have another JJ- 80uf@450, will that work or should i stay with the 40?
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Stay with the 40
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Stay with the 40
Ok, let me know if these check out-
Pre-amp-
40UF@450- I found a F&T 47 @500v
Power amp-
30/600v- I will use your suggested MOD caps- 2 -70's in series to get 35uf@ 700v
100uf@450v & 90uf@200v- I have 2 F&T 50/50 snap in's that I'll parallel each one to get 100 uf each- both @500v
Am I good to go with these?
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I suggest you to study a little with, for example, the excellent books of Gerald Weber.
Because the questions you ask are the basis of amplifier repair.
These are great books, easy to read and understand. You will like them for sure.
I have them all and many more.
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I suggest you to study a little with, for example, the excellent books of Gerald Weber.
Because the questions you ask are the basis of amplifier repair.
These are great books, easy to read and understand. You will like them for sure.
I have them all and many more.
Thanks for the book info!
For now, would you please respond about the cap list, only because I want to make the parts order so it will be here this weekend
Thanks!
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Stay with the 40
Ok, let me know if these check out-
Pre-amp-
40UF@450- I found a F&T 47 @500v. :thumbsup:
Power amp-
30/600v- I will use your suggested MOD caps- 2 -70's in series to get 35uf@ 700v. :thumbsup:
100uf@450v & 90uf@200v- I have 2 F&T 50/50 snap in's that I'll parallel each one to get 100 uf each- both @500v. :thumbsup:
Am I good to go with these?
Yes, good choices. :thumbsup:
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Thanks so much!
I'll be back after the install!
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Thanks so much!
I'll be back after the install!
Good luck my friend !
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Thanks so much!
I'll be back after the install!
Good luck my friend !
Thanks!
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As far as mounting the can caps-
The stock paper caps were held in by the insulator. I bought the metal plate , insulator and clamp. I was thinking of using the insulator that will be against the chassis and the metal plate.
Would you feel I'd need the clamp?
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Show what caps you bought
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Show what caps you bought
CE Can Cap- 100/40/450v
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The clamp is not used with those caps. You would normally use the metal plate since the cans of both caps are connected to ground. However, you may not need any mounting plates. Just use the plates that are already rivetted to the chassis if the twist lock cans will fit.
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ok, thanks. Will do that.
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removed
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Are you saying not to use the CE and get the Fliptop cap. Or could I cover the CE with a paper tube?
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I'm playing safe. I bought the insulated caps and it's matching isolator disc.
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Are you saying not to use the CE and get the Fliptop cap. Or could I cover the CE with a paper tube?
Look at the cans that are in the amp! Better yet, post a pic and we'll look for you. If your present cans have cardboard covers then you need cardboard covers (You can probably use the old covers) But if your cans are bare metal then you don't need cardboard. According to the "last" schematic you posted, both of the cans have the negative lead connected to ground. If this is the correct schematic then you don't need the cardboard.
One of your pics shows the cans mounted on a fiberglass mounting plate with a black wire connected to the tab for the can. That black wire should connect to ground somewhere.
Speaking of schematics... There have been several incorrect schematics posted in this thread. If you are sure the last schematic is correct (the cap cans match), I would like to go through the thread and remove all those incorrect schematics, with your approval of course. LMK
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I'm playing safe. I bought the insulated caps and it's matching isolator disc.
You only need insulated cans if the cans in your amp are insulated. ARE THEY INSULATED?
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The stock caps are paper covered like a toilet paper tube. I haven't removed them as yet. I was waiting for the new parts. There is a fiberglas insulator visible. I can send pics later when I get home. Last night I checked all 4 twist tabs, on each cap. Each tab grounds out with a meter. Flip top only sells insulated caps for both cap models (70/40/40 & 100/40) for the old SVT.
At the beginning of the post I sent the schematic with the 70/40/40 cap set. Mine has the 100/40 cap set.
You can remove the schematics from posts- 6,8 & 17
Posts 36 & 39 are what I have
Thanks
https://ampeg.com/support/files/Schematics/S%20Series/SVT%20(1972,%206550%20tube)/SVT%20SCHEMATIC.pdf
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removed
You were right, THANKS!!
Fliptop said that that the amp required covered filters caps due to voltage on the outside. I'm somewhat surprised that Ampeg didn't design with discreet caps, like Fender did.
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Fliptop said that that amp required covered filters caps due to voltage on the outside. I'm somewhat surprised that Ampeg didn't design with discreet caps, like Fender did.
Nothing wrong or inferior with using multicap cans. Look at all those big Marshalls, Sunns, Magnatones, etc. Lots of multicap cans. Leo probably bought train car loads of axial caps for pennies each. :icon_biggrin:
Last night I checked all 4 twist tabs, on each cap. Each tab grounds out with a meter.
The metal cans of those two multicaps are connected to chassis ground. There is no voltage on the cans. The schematic says so, I've been shouting it, and you have proven it with your meter. Those particular caps don't ***NEED*** to be insulated. However, a lot of the cans used by Ampeg were stacked in series so the cans on the top of the stack will have B+ on them. Surely Ampeg decided to just stock/use all cardboard insulated caps rather than have some insulated and others not insulated. Avoids assembly confusion/errors and probably cheaper to stock only one kind. The same goes for the fiberglass mounting plates even though your amp could have used metal. Why risk some minimum wage assembler putting a metal plate in the wrong place?
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Fliptop said that that amp required covered filters caps due to voltage on the outside. I'm somewhat surprised that Ampeg didn't design with discreet caps, like Fender did.
Nothing wrong or inferior with using multicap cans. Look at all those big Marshalls, Sunns, Magnatones, etc. Lots of multicap cans. Leo probably bought train car loads of axial caps for pennies each. :icon_biggrin:
Last night I checked all 4 twist tabs, on each cap. Each tab grounds out with a meter.
The metal cans of those two multicaps are connected to chassis ground. There is no voltage on the cans. The schematic says so, I've been shouting it, and you have proven it with your meter. Those particular caps don't ***NEED*** to be insulated. However, a lot of the cans used by Ampeg were stacked in series so the cans on the top of the stack will have B+ on them. Surely Ampeg decided to just stock/use all cardboard insulated caps rather than have some insulated and others not insulated. Avoids assembly confusion/errors and probably cheaper to stock only one kind. The same goes for the fiberglass mounting plates even though your amp could have used metal. Why risk some minimum wage assembler putting a metal plate in the wrong place?
"the cans used by Ampeg were stacked in series so the cans on the top of the stack will have B+ on them. " That's what Fliptop said
Also, Fliptop said that they are out of the 100/40 so they are subbing the 70/40/40 and I will get 110 instead of 100. He said that it's been done many times over.
I'll have pictures to show soon
Thanks
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"the cans used by Ampeg were stacked in series so the cans on the top of the stack will have B+ on them. " That's what Fliptop said
Well, some are. BUT NOT YOUR AMP! WHAT DID YOUR OHM METER TELL YOU???
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Got it.
I was just writing you to what they said
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Paper cap and insulator
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Why did you drill the rivets out? The fiberglass mounting plate should have stayed attached to the chassis. This is supposed to be easy, especially since the tabs are not soldered to the mounting plate.
The "correct" way to remove a twist-lock can is to remove any wires and solder from the twist-lock tabs. Then use your needle nose pliers to straighten the tabs. At this point the can will easily slip away from the mounting plate.
To properly install a twist-lock can simply insert the four tabs into the four slots of the mounting plate. Use your needle nose pliers to give each tab a quarter turn. This securely locks the can to the mounting plate.
FYI, those original fiberglass mounting plates are far superior to a phenolic mounting plates that you will probably get from F/T.
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Why did you drill the rivets out? The fiberglass mounting plate should have stayed attached to the chassis. This is supposed to be easy, especially since the tabs are not soldered to the mounting plate.
The "correct" way to remove a twist-lock can is to remove any wires and solder from the twist-lock tabs. Then use your needle nose pliers to straighten the tabs. At this point the can will easily slip away from the mounting plate.
To properly install a twist-lock can simply insert the four tabs into the four slots of the mounting plate. Use your needle nose pliers to give each tab a quarter turn. This securely locks the can to the mounting plate.
FYI, those original fiberglass mounting plates are far superior to a phenolic mounting plates that you will probably get from F/T.
A tech told me to drill out the rivets. I'll remove the can from the plate and re-mount.
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It's been a while since I did a cap job
I have a variac so what would be good steps to follow as far powering up?
Thanks
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I use bulb limiter. I don't think we need to that.
IMO, at the amps building plants, they fire the amps with no variac , no bulb limiter
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Success, no smoke.
All kidding aside , I used the variac, anyway, and brought it up slow. The pilot light lit up at around 90 volts. When I switched the stand by I had no noise and very minimal hum.
I found a NOS Mullard 7147/12dw7 for v1 and a NOS Sylvania 6c4 which is quiet.
The amp biased easily and the hum control took away some slight back ground hum. But, the balance control doesn't seem to do anything. All you hear is a chaffing sound when moved. the meter moves while I move the dial but still settles on .000 or .001 anywhere in the travel.
I thought the bias/ balance section was similar to a Sunn 300t, but I may need a scope to really balance it
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The amp biased easily and the hum control took away some slight back ground hum.
I thought the bias/ balance section was similar to a Sunn 300t, but I may need a scope to really balance it
With 3 pairs of tubes, you still need to do measure individual tube current and swap around the set to see which combination of which tubes on which side achieves best balance on both sides for the balancing control to work optimally.
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But, the balance control doesn't seem to do anything. All you hear is a chaffing sound when moved. the meter moves while I move the dial but still settles on .000 or .001 anywhere in the travel.
You've already set the idle bias. Now you're gonna adjust the PI balance. The instructions are on the schematic. Did you read and understand? You will need a signal generator and a 4Ω 330 Watt dummy load. Do you have proper equipment?
You will connect a signal generator to the amp input. Apply a 200mV 40Hz signal. Adjust the amp's volume until you measure 25Vrms across the 4Ω dummy load connected to J4. The amp will be putting out 150 watts of power at this point! Now connect your dmm between test points K1 and K2. Adjust the PI balance for zero VDC. You're done.
CAUTION!!! BE ABSOLUTELY SURE THE DUMMY LOAD IS SECURELY CONNECTED TO THE SPEAKER JACK J4. A POOR CONNECTION WILL LIKELY CAUSE A MELTDOWN.
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...Now you're gonna adjust the PI balance....
Disagree. Ampeg was knob-polishing. Their fussy process is correct, but IMHO it is NOT needed. Even precision High Fidelity amps don't all go through this busy-work (and the ones that do may just be supporting dealer service profit).
I had a Fisher which did have this AC Balance plan, I turned it night and day for a month with many meters, and if it was anywhere near close it was all the same.
Are we still working from the schematic in Reply # 68?
Then, with power OFF, trim VR3 for 4Kohms +/-1Kohms. Done. Have a beer.
Personally I'd rather have a 3.9k fixed resistor than an unreliable trim-pot.
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The amp biased easily and the hum control took away some slight back ground hum.
I thought the bias/ balance section was similar to a Sunn 300t, but I may need a scope to really balance it
With 3 pairs of tubes, you still need to do measure individual tube current and swap around the set to see which combination of which tubes on which side achieves best balance on both sides for the balancing control to work optimally.
Right , with a 1 ohm resistor solder at each cathode or a bias probe
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...Now you're gonna adjust the PI balance....
Disagree. Ampeg was knob-polishing. Their fussy process is correct, but IMHO it is NOT needed. Even precision High Fidelity amps don't all go through this busy-work (and the ones that do may just be supporting dealer service profit).
I had a Fisher which did have this AC Balance plan, I turned it night and day for a month with many meters, and if it was anywhere near close it was all the same.
Are we still working from the schematic in Reply # 68?
Then, with power OFF, trim VR3 for 4Kohms +/-1Kohms. Done. Have a beer.
Personally I'd rather have a 3.9k fixed resistor than an unreliable trim-pot.
Are we still working from the schematic in Reply # 68?- yes
I'll use my Bias Rite to see here I am with this set.
Question- is the balance control similar to Fender bias balance control?
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The tubes are a set of Valve Art 6550A
1st measurement-
Average voltage is 690v
measurements are in watts
1-17
2-16
3-15
4-15
5-17
6-15
Seems to be very cold
2nd after boosting the bias
1-25
2-24
3-18
4-25
5-27
6-25
Could the balance pot be bad? If it's suppose to work like a Fender, it does nothing to shift or take away from either side
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Could the balance pot be bad? If it's suppose to work like a Fender, it does nothing to shift or take away from either side
Read all bias voltage at tubes's grid.
You'll see if pot work or not.
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The cathode current avg is 35, less on #3
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Could the balance pot be bad? If it's suppose to work like a Fender, it does nothing to shift or take away from either side
Not likely. This PI balance is nothing like the Fender bias balance.
1. Did you read and understand the instructions on the schematic?
2. Did you read and understand the instructions I wrote in my previous reply?
3. Do you have the proper test equipment?
If you answer no to any of these 3 questions then you cannot properly adjust the PI balance. In that case, just follow PRR's advise...
Then, with power OFF, trim VR3 for 4Kohms +/-1Kohms. Done. Have a beer.
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Could the balance pot be bad? If it's suppose to work like a Fender, it does nothing to shift or take away from either side
Not likely. This PI balance is nothing like the Fender bias balance.
1. Did you read and understand the instructions on the schematic?
2. Did you read and understand the instructions I wrote in my previous reply?
3. Do you have the proper test equipment?
If you answer no to any of these 3 questions then you cannot properly adjust the PI balance. In that case, just follow PRR's advise...
Then, with power OFF, trim VR3 for 4Kohms +/-1Kohms. Done. Have a beer.
Looks like I'm going in the direction of the beer. I don't have the test gear.
Do you feel the tubes are still good? They are relatively close.
Bias wise, for a 6550, this seems to be on the low side at an average of 24. I was thinking of bringing them up 27-27 watts
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Looks like I'm going in the direction of the beer. I don't have the test gear.
Good idea, but first set VR3 to 4K since you've been tweaking it.
Do you feel the tubes are still good? They are relatively close.
Absolutely. You did set the two bias pots for 72mV at test points K1 and K2? When you did this you balanced the idle bias for the two sides. Nothing more to do.
Bias wise, for a 6550, this seems to be on the low side at an average of 24. I was thinking of bringing them up 27-27 watts
Nothing wrong with running like that. Ampeg thought it was a safe operating point that still produces more power than any sane person needs. A Bass amp is supposed to have low distortion, which means running the tubes cool. You don't want to buy a six pack of those tubes every six months. I highly recommend setting the bias pots exactly as Ampeg suggested. Instructions are on the schematic so a year from now you can simply readjust using the same procedure. Easy peasy.
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Looks like I'm going in the direction of the beer. I don't have the test gear.
Good idea, but first set VR3 to 4K since you've been tweaking it.
All set, the v3 is set to 4k
Do you feel the tubes are still good? They are relatively close.
Absolutely. You did set the two bias pots for 72mV at test points K1 and K2? When you did this you balanced the idle bias for the two sides. Nothing more to do.
The calibration of the back of the amp says to bias vr1/k1 with the common port to 72mv. Then you're supposed to balance with the probes in k1 & k2 to .01,using the vr2 bias pot. If I do that, then k2's adjustment goes to 83mv, if tested like the k1. Should I just go with individual testing and not balance between K1 & K2? Or use the Ampeg way?
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Some follow-ups-
The paper filter caps measure a so-
Stock was 100/40 @ 450v
They both measured
165/64
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Then you're supposed to balance with the probes in k1 & k2 to .01, using the vr2 bias pot. If I do that, then k2's adjustment goes to 83mv, if tested like the k1.
THAT AIN'T WHAT THE PROCEDURE SAYS! Read it again and again and again until you understand what it says. BTW, both procedures say exactly the same thing.
You're supposed to adjust VR2 for ZERO volts between K1 and K2. Think about it before you answer... If K1 reads 72mV and you have zero volts between K1 and K2, what voltage must be on K2? If you answer this wrong I'll never speak to you again. :icon_biggrin:
I've attached a stripped down PI with an explanation of the circuit and what happens when you adjust VR3.
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If you measure k2 and common it should be the same as k1. 72mv
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If you measure k2 and common it should be the same as k1. 72mv
OK, and if you measure voltage between K1 and K1, what will you have?
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If you measure k2 and common it should be the same as k1. 72mv
OK, and if you measure voltage between K1 and K1, what will you have?
between K1 and K1
How is that done? There's only 1 k1
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If you measure k2 and common it should be the same as k1. 72mv
OK, and if you measure voltage between K1 and K1, what will you have?
between K1 and K1
How is that done? There's only 1 k1
Now I'm going crazy too! Let me try again...
OK, and if you measure voltage between K1 and K2, what will you have?
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If you measure k2 and common it should be the same as k1. 72mv
OK, and if you measure voltage between K1 and K1, what will you have?
On the lowest scale-
I rechecked k1 and common, then k2 and common- both are at 72mv each
k1 & k2 are now .8
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I give up. I'm gonna go play with my pet rock.
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K1 and common are 72 using the vr 1 bias control
k1 and k2 are now at 0, using the vr2 bias control, just at Ampeg wanted.
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Then, with power OFF, trim VR3 for 4Kohms +/-1Kohms. Done. Have a beer.
Probably the easiest way to do this is to pull V1. Now connect one meter probe to chassis and the other to pin 3 of the V1 socket. Adjust VR3 until your ohm meter reads 15K. Done. Have a beer.
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Then, with power OFF, trim VR3 for 4Kohms +/-1Kohms. Done. Have a beer.
Probably the easiest way to do this is to pull V1. Now connect one meter probe to chassis and the other to pin 3 of the V1 socket. Adjust VR3 until your ohm meter reads 15K. Done. Have a beer.
I set it before I put the amp back together
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Any tips to lubing the fan to get it to be quieter? It has the stock fan
Thanks
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Is it squealing? Or air-noise? Cleaning the dust-scum for smoother airflow may reduce fan noise.
BUT a properly played SVT should leave no room for comments about its fan.
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The fan exterior is clean. I'd a little lube and it should be fine.
Otherwise a new one is $35 from Fliptops. I'd guess a good desktop case fan would be good too
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Is it 12VDC? Or 120VAC?
Low-price DC fans hide the bushing under the label, maybe a rubber plug under the label.
Good AC fans come all different ways.
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https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/50093-
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After watching some SVT videos, I want to replace the screen ,22ohm resistors. Viewing them again, they did look brownish and un-healthy looking. Before, one was bad and I should if replaced them all, now I'll go back and do it.
The question is , since the schematic says nothing, what value are the diodes they are connected with?
Thanks
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Many diodes have the numbers printed on the body of the diode.
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Many diodes have the numbers printed on the body of the diode.
does 555- 2 with a sides "S" mean anything?
I have some IN4007- r-SI, 1000v, 1A diodes
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Part numbers are on the schematic. According to this B15S parts list those 530555 diodes are rated 1000Vpiv and 1 AMP. A common 1N4007 has those exact same parameters.
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Part numbers are on the schematic. According to this B15S parts list those 530555 diodes are rated 1000Vpiv and 1 AMP. A common 1N4007 has those exact same parameters.
Thanks!
edit... fixed quote... sluckey
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Is 2 watt overkill for screen resistors? Is it OK to sub a 20 ohm instead of a 22?
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I'm at the parts shop and they have 15 ohm 5watt not 10 ohm. Still use?
I don't see any 10Ω resistors on the schematic.
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I don't see any 10Ω resistors on the schematic.
[/quote
Going from 5 to 10.
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They had the 22 in 1/2 watt
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I suggest using the correct parts.
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2W 22r is 0.3 Amps. Screens have no business sucking that hard long enough to toast a big resistor. Either it is just old brown/oxidized varnish, or something is wrong.
And that is 0.3A *reverse* (since the diode bypasses normal screen current).
I have my own opinion (somebody over-designed) but certainly the stock factory values should work smokelessly.
What is a "parts store"? Like NAPA to get a tie-rod for the Jeep? DigiKey will sell any likely resistor, no compromise.
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2W 22r is 0.3 Amps. Screens have no business sucking that hard long enough to toast a big resistor. Either it is just old brown/oxidized varnish, or something is wrong.
And that is 0.3A *reverse* (since the diode bypasses normal screen current).
I have my own opinion (somebody over-designed) but certainly the stock factory values should work smokelessly.
What is a "parts store"? Like NAPA to get a tie-rod for the Jeep? DigiKey will sell any likely resistor, no compromise.
Cool, I'll go stock. The part store is You Do It Electronics and has been in business for over 40 years. They have a parts floor and a audio video floor.
They stock NTE resistors in a large range of values
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They stock NTE resistors in a large range of values
Just not the values you need! :l2:
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They stock NTE resistors in a large range of values
Just not the values you need! :l2:
[/quot
Actually they checked reserve stock and I lucked out
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You need some practice with the quote button!
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Well, I finished the amp yesterday and it was quiet and awesome. I turned it on today and there's no sound and loud hum.
What do I do first?
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I found the hum. The positive signal wire fell off the square pre-amp plug.
A video I watched said that they are tricky and sometimes some tech's remake them.
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Cool, I'll go stock. The part store is You Do It Electronics and has been in business for over 40 years. They have a parts floor and a audio video floor.
They stock NTE resistors in a large range of values
Look it is a great store ; https://www.youdoitelectronics.com
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I found the hum. The positive signal wire fell off the square pre-amp plug.
A video I watched said that they are tricky and sometimes some tech's remake them.
You do a great job on this amp , congrat !
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I found the hum. The positive signal wire fell off the square pre-amp plug.
A video I watched said that they are tricky and sometimes some tech's remake them.
You do a great job on this amp , congrat !
Thanks, but I have to give much of the credit this forum.
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Tube question-
The v1 for this amp is a 12wd7. After playing his a bit , and since the volume escalates very quickly, what lower gain tube can I use?
Thanks
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> 12wd7
12DW7?? Altho I don't see one in the schematics in this thread.
> ....the volume escalates very quickly
Are you trying it in the garage? Don't! The SVT is scaled for a coliseum.
Because the volume pot is LINear it will get real loud by "4" on the dial, and be real twitchy, in a small space. When your 40,000 screaming fans arrive, you will want that leverage.
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> 12wd7
12DW7?? Altho I don't see one in the schematics in this thread.
> ....the volume escalates very quickly
Are you trying it in the garage? Don't! The SVT is scaled for a coliseum.
Because the volume pot is LINear it will get real loud by "4" on the dial, and be real twitchy, in a small space. When your 40,000 screaming fans arrive, you will want that leverage.
There should be schematic on the first page of this post. It's being used in a studio. I'm just wondering if I could get a longer swing of volume. If it's no, I'll live with it
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The v1 for this amp is a 12wd7.
Never heard of that. Schematic shows 12AX7.
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Here you go. This is my pre-amp but not my power amp. There were versions with all 12ax7's, this one is with 12dw7's
The back plate on my amp shows V9-SVT
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Schematic posted in #131
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That schematic is interesting. Not very common to see a 12DW7 in a guitar amplifier, but 4 of them? Am I interpreting this schematic correctly, that there is high voltage passing through the speaker plug connector?
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Hi All,
The correct schematic is 3 posts back
Thanks
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That schematic is interesting. Not very common to see a 12DW7 in a guitar amplifier, but 4 of them? Am I interpreting this schematic correctly, that there is high voltage passing through the speaker plug connector?
Take a look at some Magnatone schematics to see more 12DW7s. Good idea on the speaker plug.
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Picture from inside the case
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In the pre-amp there's a 40uf@450V
If you mean C12b, then you could place an 80uF cap there w/o problems, as long as it fits.
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In the pre-amp there's a 40uf@450V
If you mean C12b, then you could place an 80uF cap there w/o problems, as long as it fits.
I've already installed a 40/450. Is there an advantage with going with the 80? Plus it would fit
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I've already installed a 40/450. Is there an advantage with going with the 80?
Without going to the effort to do a real analysis, I seriously doubt there would be a noticeable (or any) advantage to a larger capacitor in that position.