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Thread: Ampeg SVT-CL chassis orientation affects plate current behaviorů.weird one!

  1. #1
    Senior Member nevetslab's Avatar
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    Ampeg SVT-CL chassis orientation affects plate current behaviorů.weird one!

    I have an Ampeg SVT-CL that came in with a tube problem. Initial observation was the left pair Bias LED was off (lower set), while right pair Bias LED was GRN (upper set). I tweaked the left bias pot to get GRN, twisted further to see RED come on, backed down to GRN, let sit there idling. Within a minute or two, the RED LED came on with GRN, then GRN went out. It was finally RED only, NO RED/GRN range on the bias pot.

    So, I pulled the two chassis out of the cabinet, and stood them up vertical with a support block under the power xfmr, and removed the six power tubes, so I could check each pair of tubes’ cathode current (plate + screen current). Before making any adjustments/checks, I first set the bias voltage on the two driver tubes to -47VDC, and checked the first pair of tubes, using V3 & V4 tube sockets in all steps. Current was too high, so decreased bias to -48VDC on each, and now had 22-23mA range. I recorded all six tubes current at this setting, finding one tube was a good 5mA greater than the other five. I also saw I could get better balance swapping V3 with V4, and made that change.

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    I next installed all six tubes, and checked current thru each from the bottom of the power amp PCB, a standard practice I use in matching and adjusting bias on SVT-CL amps. I had to settle for 26mA on the one power tube that was running hot, with the others in the 20-22mA range. After leaving it idle for a while, the RED LED came on, finding that V5 tube had climbed up past 29mA. So, backed the average current down lower, with V5 now at 24mA. That seemed to hold ok, having left it idle for some time. I next ran burst pink noise thru the system, speaker attached, to see if this one tube would misbehave. It remained stable.

    I looked thru my stash of KT-88 & 6550 pulls, looking to see if I might have any more ARS J/J Tesla KT-88’s, as that’s what this sextet was installed. I found one, tried it, and it was a bit lower in current, so I swapped it in, then rebalanced the set of tubes, and ran with it. Finally, put the amp back together, with everything appearing to be working fine, while it was still standing on it’s side in my test setup.

    Now back in the cabinet, I powered back up for final test, and, switching out of Standby, the left RED LED was lit along with the GRN LED! This is NOT what I had prior to reassembly. I tweaked the bias pot for that lower half, and found very little range between no LED, GRN and RED, while the upper half had nominal trim range on the bias pot. This is where I began a couple days ago!

    Not knowing what was the cause, I began the disassembly of the amp in steps, checking this bias state after removing the preamp from the cabinet, still connected, then removing the power amp mtg screws from the chassis, checking again, then, disconnecting the preamp, removing the power amp chassis, setting the cabinet aside and re-connecting the preamp, now power amp and preamp connected, laying flat on the bench. Still same problem. Next, I tipped the two chassis upright on the power xfmr end, with support block to keep the power amp stable. Checked again, and in THIS ORIENTATION, NOW the bias condition is working normal! I have adjustment range on the bias pot for the lower half of power tubes. Put the two chassis back down flat on the formica top bench, no adjustment range on the bias pot for the lower half output stage tubes.

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    THIS problem I have NEVER SEEN! Even when laying the amp/preamp back down horizontally, it is drawing more power from the wall then when vertical.

    I just called over to the Guitar Dept, which is down the street and around the corner, no longer conveniently being right across the street (at CenterStaging, LLC in Burbank, CA). Do you have a fresh Matched Sextet of KT-88 power tubes in the tube stash there? NO….just a single KT-88 tube. ONE. Uh-huh. 90% of our rental inventory is tube based, with lots of amps using KT-88’s. One tube???!!! I have an excuse for not having full sets of every tube needed….I’m poor. I buy food instead of tubes ‘cause I’m poor.

    I have one other issues with this amp….three of the six power tube positions are somewhat microphonic. Tap on the top of all six tubes, three of them will respond with the RED bias LED flashing to the tap, and you can hear the sound thru the speakers. I haven’t yet removed the tubes again to re-tension the forked socket terminals to get better contact. I did scrub the terminals earlier with round bristle brush and alcohol, but that’s all.

    So, I’m now waiting for the Guitar Dept to deliver another Ampeg SVT-CL, from which I’ll rob that set of tubes and plug them into here, and see if the same thing happens with respect to amp orientation. I’ve NEVER seen this orientation-based problem before. All the amps I set up are done with them vertical, and behave just fine when finally reassembled and horizontal as normally used.

    0 Not allowed! Not allowed!
    Last edited by nevetslab; 10-10-2019 at 10:31 PM.
    Logic is an organized way of going wrong with confidence

  2. #2
    Old Timer Leo_Gnardo's Avatar
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    Some 'a them tubes got saggy screens!

    And yes it IS annoying to have to personally buy inventory for an outfit that ought to stock their own. I can tell you stories ... in any case every tour I did, I had to bring my toolbox as well as a stash of tubes, fuses, and all sorts of parts. Came in handy many times, for the band gear as well as the PA/Monitor rigs. Sometimes even the lighting gear. But I always got heat from management when I submitted bills for the parts I used. Had to let 'em know, you can either do it my way, or YOU carry spares and pay for them, or go into a panic when something crashes and maybe not have a show. So... what's it gonna be? Another Rodney Dangerfield moment, no respect. Repeat ad infinitum.

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    Enjoy. Every. Sandwich.

  3. #3
    Senior Member nevetslab's Avatar
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    Today, finally having received another SVT-CL from our inventory, I removed the power tubes I had in the amp under repair, and, one by one, plugged in the tubes from the inventory amp. When I got to the last tube...V5, I discovered that tube had been broken in half! It left the Guitar Dept yesterday working, and arrived here with a broken tube! Base clamps, rather than the silicon rubber-sleeved hold-down clamps I have all of our other amps fitted with. Five tubes won't do. Sovtek 6550's installed in that amp, so I needed another Sovtek 6550 pull. I found some with Fender Groove Tube marking on it, but was Sovtek 6550 from it's label. Tried the first one, pulled too much current. Tried the second one, and was in the ballpark. So, I ran with that to see what the results were. I still had the amp being scrutinized up on its' power xfmr side, and after balancing the currents of the two halves, I then checked the two Window Comparator test points that set the low/high condition, nominally 147mV and 294mV. Both were reading 151mV and 301mV, so that was nominal.

    Laid the amp/preamp down flat, powered back up to see if I had repeat of the original symptom, where there was no adjustment range on the lower half bias control, while there WAS range when vertical. It powered up fine, tapped on all six power tubes and found with this set of tubes, NO microphonics nor flashing of the Lower half RED LED in response to tapping on the tubes. After a couple minutes of it remaining stable, I checked the adjustment range of the bias controls, lower half first, it being the one that has been off. Working just like the upper half. And, as many have found in tweaking the bias pots, using residual hum as an indicator from the speakers, the 'sweet spot' also corresponds to minimum hum. I also had the wattmeter indication to aid in that, setting back to the nominal bias setting I had dialed in.

    So, it WAS the power tubes. Just one behavior I've never seen on this amp. Now, I gotta order another set of tubes to finish this up. And, now I have to find what fool on the staff is still flipping SVT's in their road cases upside down in one muscular motion once the road case is in the truck, which breaks tubes!!

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    Logic is an organized way of going wrong with confidence

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