Ampeg SVT CL

[bsco]

Good evening....(well it's 10:48 PM here), a quick question about an SVT...Had one come in for repair and had an output tube that was after turning white and blew.....also two other tubes in the other set of three were dead..they lit up but no emission at all...so I replaced all six with a matched set.....(supposedly) and noticed that I couldn't get the amp to bias evenly on both sides. I removed the 12AU7's and tested them in a tube tester.....tested ok for shorts and emissions..... put them back in and same thing....(grove tubes) and these were fairly new.......just for the hell of it, I replaced them with a set of JJ's and the biasing was good....it was very easy to set up. I checked both of the JJ's in the same tube tester and they tested exactly the same way....but the grove tubes 12AU7's affected the biasing drastically....any idea as to what would be causing this?? I realize that it is probably the tubes themselves...but what could be wrong with them??? I also heard that the JJ's are a better choice for tubes...is this true?? If it is then maybe I should be using these in all the amps...whenever I can.....any comments......quite welcome.....
Cheers,
Bernie

[Enzo]

I don't want to sound like a smart ass, but those tubes that didn't work, what was wrong with them? They didn't work, that was the problem.

Tube testers are good at verifying tubes that don;t work well or at all, but they don;t guarantee good tubes. The conditions in the tester are not real world conditions. SO the tube may have emissions and no shorts, but that doesn;t mean they have the gain they should or can conduct full current or are otherwise operating well. A tube that is shorted or won;t emit is certainly bad. But as you found out, a "good" tube doesn;t always work.

Bad 12Au7 driver tubes are one of the most common failures in these amps. A power tube gone white is cracked and has air in it. But failed tubes in general could even have been caused by a bad 12Au7.

[bsco]

Thank you Enzo.....and no, you are not being a smart ass...I appreciate your comments. I will pay closer attention to these amps in the future....I have another one here that has a biasing problem so I will check out the drivers first...I could still have a bad output tube but I will pay closer attention to the drivers first and then take it from there. I will be looking at the other SVT tomorrow.....I'll let you know how I make out....Thanks for your help.....
Cheers,
Bernie

[Enzo]

Something else I encounter. People sticking 12AX7s in place of the 12AU7s. That doesn't work well at all. And the right tubes in the amp but in the wrong sockets.

[Prairie Dawg]

This is interesting stuff indeed. It was one of these SVT excursions that led me to build a special purpose tube matching rig last year. Operating voltages are pretty high on the SVT and it's a good idea to know that your power tubes are accurately matched, have not drifted after a few months service, and will survive working voltages. I also invested in a reference set of tubes that never leaves my shop and are matched within 3ma highest to lowest. The sticker price of the power tubes may not necessarily get you the best tubes or the ones that live the longest.

[bsco]

Thanks for the heads up Enzo...I haven't run into that problem as of yet but I am sure I will sooner or later. Thank you..Pairie Dawg......I know what you mean..I have had a couple sets of matched tubes and when I used them there was one tube that didn't match very well with the other 5.....I had to test a bunch of new singles to find one that worked with the rest...A real pain in the ass...I would like to see a pic of this jig you built....sounds like a very good idea.....
cheers,
Bernie

[Prairie Dawg]

It's a variation on the Jack Price tube matcher he described on his old website which I think is still up. PriceLess Amp Restoration

I had an email conversation with Jack and he thought the power transformer he used in his original build was a little marginal when testing a quad of 6550s. I ended up using a monster filament transformer from an old organ and an old Thordarson television power transformer that I acquired through mesne conveyances.

It was well worth the effort with a couple of caveats. If line voltage varies as it does when the utility is under stress in hot weather like we're having lately readings may drift up and down a little bit as voltage cycles. It's not a biggie but something to watch for. After the tubes have cooked for a while they seem a bit more stable. Using a tube tester to screen for shorts and such before trying to evaluate a set of tubes because a shorted tube can cause some blown plate load resistors in the test set but I bought 40 or 50 of them so I just swap them out. I start with the plate voltage low and the bias voltage high and gradually adjust until I get to working voltage and a good bias voltage setting. I usually use this test set for every amp that I service, and I have found I am considerably more picky about matching than a lot of people.

A lot of folks use the Maxi matcher and that's fine but it does not run more than 400v on the plates which as you know is low for SVT service.

I run a variac for the source which powers the plate and bias supply, and I thought I might need one for the filament supply but my filament transformer was running about 40 12AU7s in its previous life. Even with 4 6550s running at full chat filament voltage doesn't drop below 6.5 or thereabouts. I figured out a way to fine calibrate it but have not yet incorporated that feature into the test set.

There are some pics and a schematic here. Judybox Revival: Tales From The Technical Fringe: Building A Tube Matcher

[tubeswell]

It's a variation on the Jack Price tube matcher he described on his old website which I think is still up. PriceLess Amp Restoration

I had an email conversation with Jack and he thought the power transformer he used in his original build was a little marginal when testing a quad of 6550s. I ended up using a monster filament transformer from an old organ and an old Thordarson television power transformer that I acquired through mesne conveyances.

It was well worth the effort with a couple of caveats. If line voltage varies as it does when the utility is under stress in hot weather like we're having lately readings may drift up and down a little bit as voltage cycles. It's not a biggie but something to watch for. After the tubes have cooked for a while they seem a bit more stable. Using a tube tester to screen for shorts and such before trying to evaluate a set of tubes because a shorted tube can cause some blown plate load resistors in the test set but I bought 40 or 50 of them so I just swap them out. I start with the plate voltage low and the bias voltage high and gradually adjust until I get to working voltage and a good bias voltage setting. I usually use this test set for every amp that I service, and I have found I am considerably more picky about matching than a lot of people.

A lot of folks use the Maxi matcher and that's fine but it does not run more than 400v on the plates which as you know is low for SVT service.

I run a variac for the source which powers the plate and bias supply, and I thought I might need one for the filament supply but my filament transformer was running about 40 12AU7s in its previous life. Even with 4 6550s running at full chat filament voltage doesn't drop below 6.5 or thereabouts. I figured out a way to fine calibrate it but have not yet incorporated that feature into the test set.

There are some pics and a schematic here. Judybox Revival: Tales From The Technical Fringe: Building A Tube Matcher

Sorry to revive an old thread, but... This is a good idea! I have a spare PT from a Rockit 130 floating around and with that I could build a matcher that runs 2 x 6550 at a time methinks. Have you got any updates to your tube matcher since 2012 Prairie Dawg?

[Prairie Dawg]

Sorry to revive an old thread, but... This is a good idea! I have a spare PT from a Rockit 130 floating around and with that I could build a matcher that runs 2 x 6550 at a time methinks. Have you got any updates to your tube matcher since 2012 Prairie Dawg?

Wow...zombie thread.

No real updates, I've been using it right along when I need to match a proper set of power tubes or check to see if something sold as a matched set really is that close. It usually isn't.
It's well worth doing. I'd like a Maxi but I don't have the kind of money it takes to get one. I did a few blog posts on it, and the only thing I ran into a hassle with was well, two things.

First was that I had to put some plate load resistors in place to make it stable when testing KT88s and 6550s, and sometimes the power floats up and down during the summer here so you sort of have to follow it along.