3Ch Dual Recto - no volume

[warped]

Amp tech just rang me.

He said some tracks on the board had arc'd which took out some resistors (He mentioned high voltage tracks? are there HV tracks on the board) . He said it could have been something in there that bridged them (some loose solder or something)

I didn't go into anything more technical than that coz I wouldn't understand (or even know what to ask).

He had to take the board out to fix it. All tubes were fine, amp ready for pickup tomorrow ~ $200

What do you all think? Anyone want me to ask him anything while I'm there? Likelyhood it may happen again?

Thanks,
Darren

 

[domct203]

There is High Voltage on the Powertube PCB. Too bad the tech didn't take a before/after set of pics.

those 3 resistors soldered together in parallel like that look fishy to me. If the tech said he did it to "bias" up the tubes last time then that's a major fail.....the biasing resistors are on the smaller board that house all the jacks, pots and switches to the back of the amp.

It does look like the tech changed the value of the wrong resistor. Although that is where the bias circuit is located, the tech should have changed the 22K (22K1) resistor (the one above the "stacked" resistors).

The bias circuit in the 3-channel Dual Recto is not on the rear panel breakout board. The only part of the bias circuit there is the switch for selecting EL34/6L6 bias range which is in parallel with a 15K resistor.

http://www.forum.grailtone.com/viewtopic.php?f=24&t=35849&start=17

2-Channel Power Supply Schematic (bias circuit is essentially the same)

Dom

 

[warped]

Thanks Dom,
I'll ask him about it when I pic up the amp.

 

[domct203]

See if he's willing to measure the idle cathode (bias) current and plate voltage in Bold/Diode. From there we can calculate what % of Max Plate Dissipation (hot/cold) the amp is running.

Dom

 

[warped]

I just spoke to him over the phone and asked him about the piggy backed resistors (Sorry Dom, I hadn't read your message before speaking to him so I didn't ask him about the idle cahode current.

He said that he used those to match the bias to the matched set of 6L6's he put in last time (Svetlana =C=). Is matching the bias to power tubes different to adjusting the bias to run hotter/colder?

I remember with one of my other amps (Sherlock Fat Head) which had fully adjustable bias, it had a bias pot to adjust the total bias (ie, set it to 30mA, or 20mA, etc) - and an adjustable pot for each power tube which you could use to make sure each power tube was balanced or something - meant you didn't need a 'matched' set of tubes as you could set each one individually.. I thought maybe what he had done was use a matched set of power tubes, then piggy backed onto the resistor to match the set to the bias value?

(I hope that makes sense? I don't really know anything about amplifier electronics)

 

[domct203]

What the tech did was put a 220K & 47K resistor in parallel to create 39K (38,727) ohm of resistance. He then put that in parallel with a resistor in the bias circuit to adjust the amp's bias voltage to the tubes to change the idle cathode current (commenly referred to as "adjusting or setting bias"). His choice of which resistor to change was not the ideal location in the circuit though. I can't clearly make out the color codes on the OEM resistor he manipulated so I don't know it's location in the circuit.

Setting the bias voltage in a tube amplifier matches the operation of the tubes to the operation of the amp as spec'ed by the amp's designer.

Running tubes hot or cold is in reference to the tube's plate dissipation limit (watts), regardless of the amp's ideal operational point. For example, a 6L6GC is commonly rated at 30 Watts plate dissipation. Although some players run higher, most agree that to avoid failures you should not dissipate more than 70% of that 30 watts at idle (70% and over is considered running the tubes hot), or 21 watts. Running hot will (slightly) shorten the life of the tubes, and increase the failure rate of a weak tube. It can also add compression and distortion to the signal due to increased clipping. Most early (vintage) amplifiers get their distortion from both the preamp and poweramp being overdriven (volume on 10) and hot bias settings.

The Rectifier series tend to idle in the 35-45% (10-14 watt) range, which is considered running the tubes cool. The benefit to this is longer tube life and less stress on other components in the amp. This also helps to keep the output section cleaner and less compressed. With a modern high gain amplifier most if not all distortion is generated in the preamp. A clean and (somewhat) un-compressed (punchy and full) output section is ideal for this.

All these specs are considered "At Idle" specs, or when the amp is on but not amplifing signal.

Both of these bias senerios will still create an amplifier that is very capable of putting out well over their rated output power at high master volume settings.

Dom

 

[R_ADKINS80]

Huh, the resistor I swapped with a trim pot on my 2008 Triple rec was on the rear panel board, not the main board.

 

[warped]

When he set the amp up with the Svetlana winged C 6L6's he set it up for 65% dissipation - tested it once he fixed the problem and that's where it is still set.

I played the amp shortly tonight and it sounds the same as it did before the problem (which was good - I was happy with it before the volume went out).

 

[domct203]

Glad everything worked out for you.

Dom

 

[warped]

Thanks for everyone ideas on this one - the tech said it was a tricky one to diagnose (with he had the board out and all his workshop testing equipment) so there wasn't really much hope just posting pics to see if anyone noticed something suss.


Much appreciated.
Daz