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Post subject: 69 SFSR Issue with Some Older Tube Pics
Posted: Fri Aug 28, 2015 12:07 pm
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In the start of the week my 69 SR as I was playing went from loud, rich, fat sound ( as much as it can have with Vol 2.25, Treb 5.75, mid 5.5, bass 3.9, Reverb 2.5, speed 0, intensity 3 ) to thin, lows gone and not right, like if my strat went from Neck Pu to Bridge PU (fat 50s) with some drop in volume. Thought cable, went to standby swapped cable, snapped the standby and off I went, 20 min later it happened again. I than went to standby for 1/2 minute, back into play, sounded normal, later it happened again, also noticed the chaise by the by the ON OFF toggle was hot could not keep my finger on it for 10 seconds before my brain said take it off of there . Turned it off for the night.
I have posted about this amp before, not sure if I should keep adding to this one, but it is a different issue
viewtopic.php?f=12&t=47799&p=1112432&hilit=+super+reverb#p1112432
This was the results after bringing to the doctor for help in March of this year,and I have been using it about 1 to 1.5 hours a night since trouble free ( or more nights than not, I do have a little life)
He set balance and bias and checked power;
38 watts @ Clip to 2 ohms
Bias Current 34 ma
Bias Voltage -45.3V / 44.6V
High Voltage 442V No Signal/ 400V
The next night I remembered he told me my V2 Telefunken was a little micophonic ( I do not use V1 so it is empty) warmed the amp, took it off standby, took a chop stick and taped on the valve cover and heard a thunk out of the speaker as I taped, not so bad turned off the amp took out V2
Image
Went down stairs ( as I said in the player,collector, hoarder, I throw nothing of value out) to were I keep my boxes of tubes and randomly selected a handful of tubes 1st one Sylvania
Image
Put this in V2 fired it up, taped on the valve cover and pingggggg, it just kept going turned the volume down but it kept pining, fliped standby, off, Sylvania out, next up GE, if my date code is right I believe this one is 1955
Image
Back on Standby off, gave it a tap not a sound. Of course I had to mess with other things, I decided to check Bias, and plate voltage.
461VDC Plate
34MADC Bias
I came up with these using a Bias Probes from Eurotubes, the $25 one. Now correct me if I am wrong as I look at this probe it is direct to pin 3 no shunt. With my meter (fluke 179) black to common red to ma, meter set to DC auto, I get 34 for my bias, I figure I can put red to Volts set meter to DCV and this in my theory should tell me volts ( of course I turn off, pull 6l6, set meter before I test for volts after ma) and I got 461 volts. Thinking maybe this thin sound has to do with Bias to high and maybe that why it is hot to touch the back of the chaise, I than dropped the bias by 2, making it 32. Checking the plate voltage after lowering the Bias, plate voltage went up to 468 VDC
So now I Play it for 1.5 hours during this time of play; I first noticed my tone was less warm, more shimmery, crisp, and a little sterile, during this time I had intermittent tone change to the thin sound and before I could stop and go standby, tone would right itself this happened maybe 3 times. I decided that the less warm, more shimmery, crisp tone came from the GE tube, and sterile was the bias change, so I tweaked it up by ear and the sterile was gone. turned it off for the night. Last night went to play 20 minute into it thin standby flipped on and off sound was good for 15 minute went thin again. This morning I re biased set to 32.5 Plate volt 467.7 V dc and went to work. I will try it tonight.
My thought could it be V4 as this is the 2nd gain stage, maybe try the GE in there and put the telefunken back in V1 or try the RCA Black plate I have 10 of these
Image
Other thought is the 5AR4C just might be too hot and I should use a get a 5U4GB instead. This Amp is a AA1069 and has a mod of a Bias pot, it still has the Balance pot. Does this mod have anything to do with the plate voltage?
Does the Rectifier Tube set the plate voltage?
Is it possible that the new (5months old) Rudy 5AR4C has gone bad and is supplying to much plate voltage.
EDIT As far as metering goes when my tech had it he had 442 v with 34 ma, I got 34ma and 461Vdc with my meter, could our house power be different to cause the 19 volt difference or could something else have changed values with in ?
Any thoughts from the experts, before I send it back to the doctor.
Hey as I am posting Pic of tubes here some more, Date code 1943 I think
Image
I believe these GAP/R code# 322623-1 are Tung Sol NJ plant 1962 I do have a lot of these all from the 60's
Image Image
I think this is a GE kind of smudged 12AX7A
Image



Thanks
mud


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Post subject: Re: 69 SFSR Issue with Some Older Tube Pics
Posted: Fri Aug 28, 2015 1:18 pm
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While microphonic tubes are never a good thing and need to be identified and replaced, I think you've got other issues at play -- likely thermal-related and probably something other than a bad bottle. I'll wager that it's a cathode bypass cap heading south or a carbon-comp resistor in a similar circuit. They're usually a bytch to diagnose and isolate......

Arjay

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Post subject: Re: 69 SFSR Issue with Some Older Tube Pics
Posted: Fri Aug 28, 2015 2:25 pm
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Could I cause further damage or total damage by using it, do you think I should not use it and bring it straight to the tech.
mud


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Post subject: Re: 69 SFSR Issue with Some Older Tube Pics
Posted: Fri Aug 28, 2015 3:26 pm
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Probably best to get it to an amp doctor as soon as possible.

Arjay

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Post subject: Re: 69 SFSR Issue with Some Older Tube Pics
Posted: Mon Nov 16, 2015 2:44 pm
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Ok so It took a month and a half, my tech guy is quite busy and only does tube amps, lucky for me I have other crappy amps to use while I was waiting. It came home this Sunday and sounds even better :D
This is what was done;
When I gave it to him close to a year ago he put in a bias control along with the balance hum pot that was there, he has now completed;
1) Updates/upgrades to black face the drive circuit.
2) Replaced all the cathode Bypass Caps
3) Replaced a few out of Tolerance Resistors
4) Replaced Leaky Signal Caps to Output Grid
5) Minimized Noise from DC leakage on circuit Board
6) Cleaned Controls, Checked Bias and Power
7) solid state diode addition for today's GZ34/5AR4 I had him do this mod
So as of now my specs are:
44 watts @ Clip to 2 Ohms.
Bias Current 32 ma
Bias Voltage -45.5V/46.0V
High Voltage 451 No Signal/ 410 @ Clip
Using 6L6 EH Sovtek
And all this for under $100 USD definitely worth the wait

mud


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Post subject: Re: 69 SFSR Issue with Some Older Tube Pics
Posted: Mon Nov 16, 2015 3:35 pm
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For a mere C-note, you got off relatively easy.

I hope the amp now remains issue-free for several years.

Arjay

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Post subject: Re: 69 SFSR Issue with Some Older Tube Pics
Posted: Tue Nov 17, 2015 8:50 am
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Retroverbial wrote:
For a mere C-note, you got off relatively easy.

I hope the amp now remains issue-free for several years.

Arjay


Yes thanks.
Now to go through my boxes of 12ax7,and 5751 I feel that Leo's design made it possible to listen to your preAmp tubes using only V1 and V6 that way you can pick the tube tone you like, almost a type of tube tester.
mud


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Post subject: Re: 69 SFSR Issue with Some Older Tube Pics
Posted: Tue Nov 17, 2015 11:15 am
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Quite true. V2 through V5 are only required for the "vibrato" channel -- thus for determining the tone and timbre of a first/second gain stage tube, using the V1 socket to "audition" a bottle makes perfect sense. Just be sure to avoid using long-plate 12AT7's in the phase-inverter position. They have a tendency to become microphonic when used in portable audio gear.

Arjay

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Post subject: Re: 69 SFSR Issue with Some Older Tube Pics
Posted: Tue Nov 17, 2015 11:23 am
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Retroverbial wrote:
Quite true. V2 through V5 are only required for the "vibrato" channel -- thus for determining the tone and timbre of a first/second gain stage tube, using the V1 socket to "audition" a bottle makes perfect sense. Just be sure to avoid using long-plate 12AT7's in the phase-inverter position. They have a tendency to become microphonic when used in portable audio gear.

Arjay


Arjay;
Thanks for that tip, as I have long and short plate 12AT7's
mud


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Post subject: Re: 69 SFSR Issue with Some Older Tube Pics
Posted: Tue Nov 17, 2015 1:02 pm
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That same caveat applies to 12AX7's as well......avoid the long-plate bottles.

Arjay

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Post subject: Re: 69 SFSR Issue with Some Older Tube Pics
Posted: Fri Nov 20, 2015 11:00 am
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This one looks like a short plate, GE 12AT7WA 6201 it registered 88/92 out of a 100, I will try this in V6 over the weekend, and start there.

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mud


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Post subject: Re: 69 SFSR Issue with Some Older Tube Pics
Posted: Fri Nov 20, 2015 12:39 pm
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+1

If it doesn't work out as a phase inverter, you can always try it in the reverb circuit where balanced transconductance is less of a factor.

Arjay

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