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Old 22nd June 2013, 03:55 PM   #11
Renioo is offline Renioo  Poland
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I have passive pickups, so nothing to worry about battery I ordered a set of 6n2p in fact but buying EL's would be pretty a pain (having holiday and only spending money ). I luckily found a producer of amp so after a contact propably I will have a schematic or any other info.
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Old 22nd June 2013, 07:25 PM   #12
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Did anyone notice the bridge rectifier in the heater lines between the power tubes and the phase inverter? I don't see a filter cap anywhere for the preamp heaters. What DC volts do you measure between pin 9 of one of the preamp tubes and pins 4 and 5? What does each side measure to ground?
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Old 24th June 2013, 09:46 AM   #13
Renioo is offline Renioo  Poland
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3,15V on both pins, so it works on 6,3V accurately
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Old 24th June 2013, 04:31 PM   #14
Struth is offline Struth  Canada
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Hi Guys

It is still disturbing that you have not measured the screen voltage and that you report twiddling grid bias to -12V. If Vs is 280V, then -12V on the grid should red-plate the output tubes.

I told you how to bias the tubes above.

Because the "problem" appeared right after you moved the amp, the FIRST suspect is tubes. Tubes will last forever electrically provided they are not mechanically upset.

Feeding pulsating DC to the heaters is asking for increased possibility of noise intrusion from the heaters to the grids due to the discontinuities of conduction caused by the rectifier. These can be smoothed using resistors, or eliminated by adding a filter cap.

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Old 24th June 2013, 06:29 PM   #15
Renioo is offline Renioo  Poland
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Screen voltage is 111V.

There are caps and resistor on the bridge rectifier.

Yes, finally i decided to buy a pair of new matched EL34 and set of NOS 6n2p-EW for amplifier.
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Old 24th June 2013, 06:38 PM   #16
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Quote:
Originally Posted by Renioo View Post
3,15V on both pins, so it works on 6,3V accurately
3.15VAC on each side does not tell us what the DC voltage is between pin 9 and pins 4 and 5. The AC voltage reading suggests that the heater winding has a center tap that is grounded. Hopefull the builder did not ground the minus side of the bridge. That would probably damage the bridge or the transformer.
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Old 24th June 2013, 06:40 PM   #17
Struth is offline Struth  Canada
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Hi Guys

How is the 11Vs created? From a separate winding on the PT?

This is very low and may restrict how much output power is attained using a standard splitter. It does howver explain how you can set the grid voltage to -12V without red-plating.

Matched tubes are a waste of money - others will disagree. It is far better to have separate bias adjustments for each tube so they can be set to the hum-balance point of the OT. Tubes drift with age, especially in a combo amp, and matched tubes will rift apart.. Separate bias also allows using other tube types and to mix tube types.

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Kevin O'Connor
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Old 24th June 2013, 09:09 PM   #18
Renioo is offline Renioo  Poland
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I think I would make a separate bias adjustment after having this amp sing back again. I know it's better.

I think the voltage can be so low because it was meant to give power around 20W, not 50W common in 2xEL34. However I can be wrong. The 115V comes from the "psu board" seen in the left top corner. (The another point on board gives plate voltage of 560V.) Then the voltage goes through, I think choke and then to grid. It also supply the anode voltage of preamplifier.

Preamplifier uses DC heaters, it's 6n2p (no center tap) and beetween 4 and 5 pin there is 6,5V.
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Old 24th June 2013, 11:21 PM   #19
Struth is offline Struth  Canada
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Hi Guys

Plate or screen voltage being low is fine. The problem is you've posted a layout that does not match your PSU. Maybe you should trace it?

If the original PT provided 50W with screens at a more usual voltage, it may be possible to power the whole amp from the 280V CT and have quarter power - about 12W. Have you verified the output power as the amp is wired now? or are you just happy that it is may be tolerable?

The Marshalls that had 560V for the plate and screen usually had a 100V winding for bias. I assume this is being used for the screens in your amp. If they used half-wave rectification from this winding, you get a low voltage - as you have - but can also still generate the bias voltage from it.

When setting idle current, remember to use the 560V as part of the calculation, since this is the plate voltage in your amp. Actually, did you measure 560V at idle on pin-3 of either octal socket, or just at the supply? It is more telling at the socket itself.

Be careful not to let the meter lead slip and short the tube pin to chassis.

Some of the wiring is pretty lousy - certainly not low-noise promoting. More like a Diaz amp...

Have fun
Kevin O'Connor
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Old 24th June 2013, 11:45 PM   #20
Renioo is offline Renioo  Poland
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It was of course measured at pin-3. I traced every voltage up to power section, just to be sure if everything is ok, and it is. I have found the producer and asked for a schematic and some information, maybe I would know something more.

I use very sensitive speakers, around 101dB/1W/1m and amp itself is dead silent. Guitar is a pretty noisy stuff and with it is ok, even better than solid-state Hartke I used before. I don't think there is a need to twist the wires to the limit.

The output power is OK with me, using 50W at home with guitar cabinet is just a problem. Now I'm waiting for the tubes, every set is already sent

First I want it too let it play correctly, then there will be time to make any mods, corrections and so on.

I'm very happy you guys try to help
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