Amp Guts Photos - Official Thread - Lets see those pics

[Passinwind]

Is it going to be for sale in small batches?

I can't see doing Class D amps for sale, too much regulatory hassle. Besides which, I'm just not a production or marketing oriented kind of guy. This build may end up being for active basses only, even, and that wouldn't be the first time.

 

[BassNugget]

Original version of this one was posted earlier in the thread.

Cheers,
Duff2

View attachment 1398077

Deffo one of the best sounding amps I have ever heard!

 

[Jeff Scott]

Main board retrofit nearly ready to go into my 2015 DIY rack format amp build. Final component selection pending an opamp shootout in the next day or two.

View attachment 3815490

A major improvement would be a couple of Vargas girls hangin' around the board.

 

[Passinwind]

A major improvement would be a couple of Vargas girls hangin' around the board.

That's a nice interpretation of "rack format."

 

[BassmanPaul]

That's a nice interpretation of "rack format." View attachment 3815975

And they could get their jollies by sitting on the subwoofers!

 

[Jeff Scott]

And they could get their jollies by sitting on the subwoofers!

In Off-Topic there was/is a discussion going on about riding horses (side-saddle, IIRC).........................

 

[agedhorse]

In Off-Topic there was/is a discussion going on about riding horses (side-saddle, IIRC).........................

Hmmm, possibly for the other gender

 

[BassmanPaul]

It gives a whole new meaning to 'Horsing around!'

 

[Jeff Scott]

Hmmm, possibly for the other gender

It was she who started the whole thing............

 

[SherpaKahn]

Ampeg B18N. Picked this one up cheap D.O.A. with a dead bias circuit and severely mismatched original 7027A. This one has the AC-coupled concertina PI. The dead bias cap was right next to the 10W screen dropping resistor/toaster.

...Working fine now, but some annoying crossover distortion that won't bias out before redplating a pair of 30W modern 6L6GC. 505VDC on the screens @4.3K plate load might be the culprit; zener droppers to shift the screen supply down 45V or so might help. Or perhaps the PI balance is outta whack. It vexes me. Sounds great at higher SPL, but the crossover distortion kills the recording-volume sound quality....for now. Plenty of 600V snap-in caps around to replace the main electrolytics down the line.

Bogen M120 guts coming soon...

[edit: gotta @beans-on-toast here. 47k on the anode, 10k/39k on the cathode of the 7199-T PI. AC-coupled with some attempt at a balance matching scheme. Do you think it's the screens, or the PI? Ampeg seems to have shifted the PI design in ~3 different versions of the B18N. Hope yer well in these times.]

 

[rufus.K]

Its this piece of paper vibrating

 

[SherpaKahn]

View attachment 3819555
Its this piece of paper vibrating

I wish I wish I wish.

But if the paper is the culprit, why does the sound artifact (crossover) diminish as I biased the tubes glowier and sparklier?

[the paper was there to keep me from melting those white twisted leads with my soldering iron as I swapped in that blue electrolytic cap. I took the pic when I did the repair several months ago.]

But you still made my day there. I hope the guinea pigs of modern telemedicine are luckier than my amplifiers. STILL LOVE YA TALKBASS! <3

edit: and @rufus.K the really messed-up fact is that some low-level crossover distortion REALLY DOES SOUND LIKE PECULIARLY RATTLING PAPER!!

 

[agedhorse]

That radial cap is available in the more proper axial style.

Regarding crossover distortion, are you sure it's the power tubes? Have you looked at what's going on before the power tubes with a scope?

 

[SherpaKahn]

That radial cap is available in the more proper axial style.

Regarding crossover distortion, are you sure it's the power tubes? Have you looked at what's going on before the power tubes with a scope?

Thanks @agedhorse for commiting some of your brain's uptime to my conundrum.

First off: the line-out signal is glorious and unencumbered by the issue. It's not in the 6SL7s. So the 7199 or the poweramp.

The B18N hasn't been on the scope yet. I tacked in some 1R cathode resistors on each 7027 socket, soldered a shunt on the bias supply set resistor (last bleeder to ground in bias RC chain, and swapped/swept through a bias range of -59V (stock, but modern increased wall voltage), so ~-59 down to maybe -40V.

...I forget the numbers off-hand, but as I measured them, the screens didn't seem to sip much current at idle, so I fudged the cathode current down to ~91% to estimate plate dissipation, and swept the plate dissipation up past 32W/well into class-A silliness on the JJ 6L6GC when they started redplating. Tried some Sovtek 6550 in there (tube sockets way too close for long-term use), and the crossover fuzz was still there, albeit with their different bias range. I have 8 of the "Super 6BG6G" top-cap 7027 available, but that seems blasphemous.

...Not the same as a scope, but I did desolder the bias feed resistors on the power tubes and measured them and the PI anode/cathode resistors, which were all within spec. Same for the big 100K dropper in the bias supply.

Thanks again. I will put this on the 465B scope and dummy load once this dummy is sober tomorrow. add/compare the PI outputs, and check the speaker output for them notches. And then I can start my "5C0pE 4R0n!!@~" thread! And you all invited.

edit: w/r/t axial cap: yes, I put in a huge mouser order today, and a couple of pricey-ish axial 100uF/100V 100uF/200V were in there for this one. The radial was what was in my cheap-and-lots-of-values stockpile at the time

 

[beans-on-toast]

Ampeg B18N. Picked this one up cheap D.O.A. with a dead bias circuit and severely mismatched original 7027A. This one has the AC-coupled concertina PI. The dead bias cap was right next to the 10W screen dropping resistor/toaster.

...Working fine now, but some annoying crossover distortion that won't bias out before redplating a pair of 30W modern 6L6GC. 505VDC on the screens @4.3K plate load might be the culprit; zener droppers to shift the screen supply down 45V or so might help. Or perhaps the PI balance is outta whack. It vexes me. Sounds great at higher SPL, but the crossover distortion kills the recording-volume sound quality....for now. Plenty of 600V snap-in caps around to replace the main electrolytics down the line.

View attachment 3819489

Bogen M120 guts coming soon... View attachment 3819494

[edit: gotta @beans-on-toast here. 47k on the anode, 10k/39k on the cathode of the 7199-T PI. AC-coupled with some attempt at a balance matching scheme. Do you think it's the screens, or the PI? Ampeg seems to have shifted the PI design in ~3 different versions of the B18N. Hope yer well in these times.]

Tung-Sol reissue 7027A’s are good tubes with better plate and screen dissipation. An option worth considering and where I would start. It is better to use the specified tube rather than trying to drop the plate and screen voltages to satisify the 6L6GC's.

Tung-Sol 7027A Audio Tubes - www.thetubestore.com

The PI balance and signal quality should be checked with a scope and a signal generator. Often PI’s are not perfect, a PI can be a source of distortion. Some imbalance provides grit if that’s what you are looking for. Check the PI component tolerances. Ampeg was always tweaking their designs. Who wants warranty returns. Sometimes they would exceed the maximum PI plate voltage. Lowering it helped increase the tube service life. Other tweeks were adding small caps to tame oscillations. As was mentioned by Andy. It may not be crossover distortion. A spectrum analyzer can help here to see if the distortion is a a high frequency artifact.

Attached is a schematic with reference voltages. Note that the references are with a 117VAC power line voltage. Read the voltage at the wall. If it's higher, the power supply nodes will be higher. 505VDC on the screens is not bad with a 7027A.

 

[agedhorse]

I think the line out jack is sourced before the 7199, but note that there's global feedback around the 7199 before the PI stage and if there's any offset or leakage, this can upset the circuit at it's applied directly to the cathode. Generally this will cause asymmetrical clipping, BUT it can also cause other unusual static conditions.

Also, I don't know if the power transformer can support a pair of 6550's.

 

[beans-on-toast]

The ext speaker jack needs to be isolated from the chassis, otherwise there is hum. Inspect that the washers are in place and in good condition. If they need to be replaced, use Switchcraft S1029 and S1029 flat and shoulder washers.

 

[beans-on-toast]

Another schematic with reference voltages.

 

[SherpaKahn]

Tung-Sol reissue 7027A’s are good tubes with better plate and screen dissipation. An option worth considering and where I would start. It is better to use the specified tube rather than trying to drop the plate and screen voltages to satisify the 6L6GC's.

Tung-Sol 7027A Audio Tubes - www.thetubestore.com

The PI balance and signal quality should be checked with a scope and a signal generator. Often PI’s are not perfect, a PI can be a source of distortion. Some imbalance provides grit if that’s what you are looking for. Check the PI component tolerances. Ampeg was always tweaking their designs. Who wants warranty returns. Sometimes they would exceed the maximum PI plate voltage. Lowering it helped increase the tube service life. Other tweeks were adding small caps to tame oscillations. As was mentioned by Andy. It may not be crossover distortion. A spectrum analyzer can help here to see if the distortion is a a high frequency artifact.

Attached is a schematic with reference voltages. Note that the references are with a 117VAC power line voltage. Read the voltage at the wall. If it's higher, the power supply nodes will be higher. 505VDC on the screens is not bad with a 7027A.

I think the line out jack is sourced before the 7199, but note that there's global feedback around the 7199 before the PI stage and if there's any offset or leakage, this can upset the circuit at it's applied directly to the cathode. Generally this will cause asymmetrical clipping, BUT it can also cause other unusual static conditions.

Also, I don't know if the power transformer can support a pair of 6550's.

The ext speaker jack needs to be isolated from the chassis, otherwise there is hum. Inspect that the washers are in place and in good condition. If they need to be replaced, use Switchcraft S1029 and S1029 flat and shoulder washers.

Thanks for the 7027A reccomendation. 3 of my other amps were made for 7591 & 8417 tubes, which has driven me to pessimism wrt sourcing supplies vs. adapting for available tubes. To be fair, 7027 aren't as twitchy as those other ones.

@agedhorse Vintage Blue service manuals put the 6.3V winding @5A on the PT-12XA. good news for the B25B crowd (diff transformer; same spec) wrt 6550, but no dice with 3/16" between glass envelopes on the B18N.

@beans-on-toast good tip on the PI signal check. I've already measured the components as up-to-spec (doesn't seem to vary with warmup temperature/extended runtime), but I'll check the signals once I'm HV-worthy tomorrow. A minimally invasive option seems to be to switch it to fixed-bias cathodyne and see what happens (if the PI signal outs are "dirty" from the artifact). I've seen examples of cathodyne balance trimmers that might be alligator-clippable to test the concept.

For now I'll also whip up a pair of socket adapters for the 6BG6 35W 7027-oids and order a pair of those Tung-Sols. The cage will never close with those adapters in place, and converting to top-cap tubes with only ~300 left at SND tube sales seems
ridiculous.

[edit: insulating fiber washers are all-good. I was baffled by the speaker jack wiring until I noticed the isolation.]

 

[beans-on-toast]

AES sells an adapter to replace the expensive 7199.
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