Tube Amps / Music Electronics
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|8/31/1997 12:43 AM|
Sorry for the upcoming long post, but I'm trying to catch up on several lapsed projects...
1. Tube tester questions--I've just started using a Hickock 533A tester, and the 9-pin socket won't light up a tube (although the octal socket works fine). When I bought the tester at a Hamfest recently, the seller gave me a replacement transformer that he had, although he said the tester worked fine. Anyway, before I open it up, any suggestions on what to look for? I've already cleaned and tensioned the socket.
Also, should the line voltage be adjusted under load (tube in socket) or not? The test results vary by several hundred mho's depending on whether or not there's a tube in. I would ASSUME that you'd set it with the tester NOT under load, but then what do I know???
2. Fender tweed Deluxe--I get an annoying overtone that sounds almost like a telephone dial tone when I play a high F# at the 14th fret on the first string of my guitars. There's also some nasty crossover-sounding distortion one fret up at the G. Elsewhere on the fingerboard I get the traditional tweed overdrive but no odd-sounding harmonics. I've duplicated the problem with two different guitars. I immediately suspected a microphonic preamp tube (notoriuos 12AY7) but that's not it... in fact none of the tubes are the least bit noisy. I've changed them all out with known good tubes to make sure. Ditto with the speaker (a relatively new Mojo MP12R) It really sounds like some kind of mechanically-induced vibration, but I'm at a loss to find it. The chassis is out of the cabinet on the bench and the problem persists.
BTW, all the electrolytics in the amp have been replaced with Spragues, there's one replaced volume pot (mic channel), and I've added Weber's line-out mod (2 resistors) to the external speaker jack, but other than that the amp is stock electronically. Would the amp benefit from replacing all the bypass caps as well?
Hope somebody can help! All ideas appreciated.
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|9/1/1997 6:55 AM|
I'll try this again - my earlier posts seem to have been eaten.
(1)It's likely that the problem is in the massive switching matrix that assigns filaments to any pin as needed. Clean the switches with contact cleaner and lots of motion. If that doesn't help, use a meter on the AC at the transformer to see if it's really making filament voltages and then track them through the wiring nest to find out where they're being lost before you start replacing the transformer.
I adjust AC line unloaded. I don't know if that's universal to all meters or not.
(2) Sounds like it's almost oscillating and a note of just the right frequency sets it off, like a high Q filter - assuming you really have eliminated mechanical. You say "all the electrolytics in the amp have been replaced" and also "would the amp benefit from replacing all the bypass caps as well?". I always though the bypass caps were electros. We're talking the caps that are on the b+ resistor dropping string, right? If they haven't been replaced, do so. If they're all new, it's possible one of them is defective new - it does happen. Try bridging a new 20uF/450V cap across each of the bypasses in turn to see if thie modifies the oscillation.
Did the problem start immediately after one of the mods?
Other than that, are the DC voltages on the tubes about right?
|9/1/1997 8:33 PM|
RG- Regarding the Deluxe... duh... I wasn't thinking very clearly (or looking at the schem) when I posted in the wee hours the other night... Yeh, I've replaced the BYPASS caps--I meant to ask about any benefit from replacing the COUPLING caps... duh...
But in the meantime I've found a toasted and cracked resistor--the 5K between the first and second filter caps. Must replace it before doing anything else. I started to measure the voltages the other night but realized that I don't have the spec data (all I have are the original Fender schem and layout--no voltages). After reading your post tonight I started to go ahead and check them anyway, to check against the tube characteristics, and that's when I found the resistor. I'll report back when I've replaced it.
BTW, thanx for the suggestions on the tester. I'll get to IT sooner or later!!!
|9/1/1997 9:23 PM|
Coupling?? It wouldn't hurt to test them for leaky. This probably isn't the motorboating/oscillation though.
Your problem MAY go away with the toasted resistor.
|9/6/1997 9:37 AM|
Well, I changed the power supply resistor, and got no change. BUT-- I finally got around to checking the power tube voltages (this week has been real busy) and they seem way high--516v on the plates and 486v on the screens. I used a couple different NOS 5Y3GT's just to be sure. Isn't that seriously high for 6V6's? Could it be that parasitic could be induced around that center frequency (I've decided it's high G instead of F# on the first string) that wouldn't be audible if the tubes weren't running so hot? Sorry for the stupid questions...
|9/6/1997 1:35 PM|
516V is WAY high. You got some solid state diodes hidden in there in parallel with the rectifier tube???
Maybe the 6V6's are just biased so cold that they can take it, but that ought to cause it to sound gritty or strained from the crossover distortion you'd get. I would expect them to be cooking..
It's possible that you are getting some kind of incited parasitic, I guess. I think it's time to get out the oscilloscope and quit playing around. Get a scope on it and see if there really is a burst of nasties when you hit the note; better yet, run a sine wave in and see just what excites the "resonance".
|9/6/1997 2:07 PM|
Okay--I've already tried that with a square wave but I guess a sine wave would indeed make more sense (I saw a little crossover on the square wave but it didn't vary much with frequency).
In the meantime, do I need to replace the power trans in this thing?? Or can I maybe put some dropping resistors in front of the center tap of the OT and the screen resistors, to lower the plate voltage? Guess I ought to check voltage on the preamp and driver plates, too; the whole thing's probably baking. I haven't measured the current draw yet on the output tubes (I'll do that next) but the amp sounds real, real good on "scream" (anything above about 2
In case I haven't mentioned it, with the re-tweed I did (dang, it looks good...), re-capping, new handle, etc. (although the grille cloth is original!!) this amp is far from being "original" so I want it to be the best playing amp it can be. If a new PT is in order, so be it... (it's remarkable that the original OT has held up, I guess, given the high plate voltages (or not?))
Anyway, in case I haven't said it lately, thanx for your help (as always)... I'll check back in later.
Regards, John E.
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