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Old 11th August 2011, 12:00 AM   #1
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Default Is my 6L6 amp damaged from EL34's

Hello, I bought a 6L6 amp with EL34's installed. I just found out that no conversion mods were done to it. I've only used it a few hours at low volumes and I don't think the previous owner used it much either after his so called tech switched to EL34's. It sounds ok but rather than do the mods I'm going back to 6L6's. I'm concerned if the transformer or anything else might've been damaged by not doing the mods. Mainly pin 1 to 8 jumper and bias adjustment. Thanks for any replies.
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Old 11th August 2011, 12:33 AM   #2
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Quote:
Originally Posted by Clemford View Post
It sounds ok but rather than do the mods I'm going back to 6L6's. I'm concerned if the transformer or anything else might've been damaged by not doing the mods.
If it sounds ok, I strongly suspect that the amp is still in good shape. If the output transformer was damaged, you'd definitely know about it. One thing you could (or should) do is carefully examine all the resistors around the output tubes and power supply area for any signs of overheating or discoloring. Replace any that look stressed. But chances are that you won't find anything serious.
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Old 11th August 2011, 12:56 AM   #3
kevinkr is offline kevinkr  United States
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Old 11th August 2011, 01:23 AM   #4
alexg is offline alexg  Philippines
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I have used 6L6 and EL34 on the same amp (Mikael Abdella's circuit) with no problem.
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Old 11th August 2011, 02:05 AM   #5
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Originally Posted by HollowState View Post
If it sounds ok, I strongly suspect that the amp is still in good shape. If the output transformer was damaged, you'd definitely know about it. One thing you could (or should) do is carefully examine all the resistors around the output tubes and power supply area for any signs of overheating or discoloring. Replace any that look stressed. But chances are that you won't find anything serious.
A couple of the resistors right off pin 8 look a little discolored but measure ok. Not sure if I need to change them . Most everything else looks pretty good. When I first got the amp there was a burnt resistor that I think went from the standby switch to ground. I think it's a bleeder resistor. I went to a higher wattage and it was ok after that. One thing I noticed and I'm not sure if it's normal considering it didn't have the mod was that the EL34's seemed to throw off far less heat than some 6L6's that I put in.
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Old 11th August 2011, 05:16 AM   #6
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Originally Posted by Clemford View Post
A couple of the resistors right off pin 8 look a little discolored but measure ok. Not sure if I need to change them .
Looking at the pictures, I'd change the discolored ones even though they measure ok just on general principal. Especially the one in the lower left corner (in pix). I'm fussy that way.

Quote:
One thing I noticed and I'm not sure if it's normal considering it didn't have the mod was that the EL34's seemed to throw off far less heat than some 6L6's that I put in.
Possible reasons for this are: A) The EL-34s were a little more sensitive G1 wise and required slightly less negative bias voltage then 6L6s. So they may have been drawing less current and running cooler. B) Todays modern 6L6s may just draw more current because of their looser manufacturing tolerance.

As I am unfamiliar with your amp, I would suggest measuring the tube current, probably by measuring the voltage across the cathode resistor (pin 8). EśR will give the current. You typically want about 35 to 40mA at most through the tube. Whether you have fixed or self bias will determine what needs to be done to adjust it.

Another helpful test is to look closely at the tubes in a darkened room. Observe that the screen grids are not glowing red. Sometimes that's hard to see depending on the tube construction. But it's a good test to do.
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Old 11th August 2011, 05:35 AM   #7
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Quote:
Originally Posted by HollowState View Post
Looking at the pictures, I'd change the discolored ones even though they measure ok just on general principal. Especially the one in the lower left corner (in pix). I'm fussy that way.


Possible reasons for this are: A) The EL-34s were a little more sensitive G1 wise and required slightly less negative bias voltage then 6L6s. So they may have been drawing less current and running cooler. B) Todays modern 6L6s may just draw more current because of their looser manufacturing tolerance.

As I am unfamiliar with your amp, I would suggest measuring the tube current, probably by measuring the voltage across the cathode resistor (pin 8). EśR will give the current. You typically want about 35 to 40mA at most through the tube. Whether you have fixed or self bias will determine what needs to be done to adjust it.

Another helpful test is to look closely at the tubes in a darkened room. Observe that the screen grids are not glowing red. Sometimes that's hard to see depending on the tube construction. But it's a good test to do.
I think I'll change out those resistors for sure. I found this partial schematic on another forum but I'm not sure if the poster changed it or not. He seemed to think there was something wrong with the circled red part and he said he redesigned it but as far as I can tell it's the mono 60 watt version of my rt-4250 amp . If this schematic is not altered where should I measure to find the bias voltage?
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Old 11th August 2011, 07:42 AM   #8
tricomp is offline tricomp  Denmark
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Hi,

There's a couple of odd things in this schematic.

The red circled part isn't a 'part'; it's a connection and it is definitely not supposed to be there. It will short out the power-transformer winding. Just delete it, and the power-supply will be fine.
You measure the bias at the center pin on the driver-transformer secondary winding. Also, if you don't know where that is, find the 1N5261 Zenerdiode (47V/500mW) and go to the right where the 10K resistor meets with the 47uF electrolytic cap. Here you should measure pretty close to -47V which will be your bias. Not adjustable as the circuit is but easily changed if you want the option.
The 470 Ohm 'bleeder' resistor attached to the Stand-by switch is really in for it when you throw the switch to stand-by. It discharges the full C+ (458V from 2 series-couples 100uF electrolytic caps, totalling 50uF) in one hell of a go. No wonder the original resistor gave-in. Isn't 470 KOhm more reasonable as a bleeder?

User-manual for the amp. attached.

rgds,

/tri-comp
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Old 11th August 2011, 08:16 AM   #9
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Quote:
Originally Posted by tricomp View Post
Hi,

There's a couple of odd things in this schematic.

The red circled part isn't a 'part'; it's a connection and it is definitely not supposed to be there. It will short out the power-transformer winding. Just delete it, and the power-supply will be fine.
You measure the bias at the center pin on the driver-transformer secondary winding. Also, if you don't know where that is, find the 1N5261 Zenerdiode (47V/500mW) and go to the right where the 10K resistor meets with the 47uF electrolytic cap. Here you should measure pretty close to -47V which will be your bias. Not adjustable as the circuit is but easily changed if you want the option.
The 470 Ohm 'bleeder' resistor attached to the Stand-by switch is really in for it when you throw the switch to stand-by. It discharges the full C+ (458V from 2 series-couples 100uF electrolytic caps, totalling 50uF) in one hell of a go. No wonder the original resistor gave-in. Isn't 470 KOhm more reasonable as a bleeder?

User-manual for the amp. attached.

rgds,

/tri-comp
I should've mentioned my Boston roots when I wrote that part of the circuit. Or should I say, paht. I can see where that line would short out the lower half of the transformer. Obviously must be a misprint. The person that posted that schematic said he rebuilt his amp using that circuit sent from Tubeworks and fried his amp and tubes. They wanted to sell it to me for 25 dollars when I asked them for one. The tech said to change the 47v zener to a 39v in4754 and up the wattage of the 10k resistor as well as jump pins 1 and 8 if I wanted to go with EL34's. I'm sticking with 6L6's. I think they sound deeper and fuller personally. I think I replaced the bleeder resistor when I first noticed it with a 200k 2 watt resistor that I had and it seemed ok after that. I didn't have 470k. The original one was 470 ohm 1/2 watt . Must've fried fairly new. Thanks by the way for looking into the manual. I got one way back on their website. I wish I could get the right schematic for the rest of the amp. One thing I was wondering was why the amp sounded so good with the eL34's even though the mods were not done. Also, I wonder if there is any way to put a pot in there to variable adjust the bias.
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Old 11th August 2011, 06:10 PM   #10
taj is offline taj
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Quote:
Originally Posted by Clemford View Post
One thing I was wondering was why the amp sounded so good with the eL34's even though the mods were not done.
I'm no guru but...

The most physically significant difference between the two tubes is the lack of internal connection between the G3 and cathode. So the EL34 would be running as a tetrode, rather than a pentode, if that connection wasn't made externally.

Besides that, the operating point(s) will determine how well suited the EL34 would be to run in that amp, but the tube specs are not horribly different in any fairly normal amp circuit. I would expect the amp to work fine, but certainly not optimally.

Guitar players make that swap every day. Naturally, they aren't concerned too much about fidelity and cleanliness, nor are they concerned about the operational distortion profile, other than whether they like the sound of it. And of course guitar amps are probably built with g3 and cathode wired externally to accommodate the expectation that many guitar players love to just "plug it in and see what happens."

..Todd

Last edited by taj; 11th August 2011 at 06:16 PM.
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