Having trouble with a selmer treble 'n' bass MKIII (high bias value)

[bruno]

I have an old selmer MKIII treble 'n' bass for repair... the transformers look like old radiospares, similar to the ones used in early JTM45s. The amp uses el34s, but there is an issue. It powers up fine, but then the readings on the bias current go through the roof, like higher then 120ma, it's literally eating up power tubes.

The values in the amp are really similar to the ones found on the original schematic:

http://www.vintagehofner.co.uk/selmer/schematic/tnb1.html

And everything else seems to be fine.

However, I  installed a pot to regulate the bias, where the 39k resistor is, I added a 50k pot in series to ground, with no big change in the bias values.

Should I check for any other reason why the bias should be so high?

[jjasilli]

Where are you measuring current?  There's plenty of current available to the bias circuit as it's tapped off the HT winding.  But the bias current should make a closed loop from: HT > parallel dropping resistors > 39K resistor (or pot) > ground.  Bias current should not reach the mixing resistors or tube pins.  Only bias voltage should reach the mixing resistors > tube pins.  

BTW:  What is your bias voltage?

1.  Are you sure your bias circuit has a ground connection?  That should be at the bottom of 39K resistor (or pot).

2.  You should have a resistor under the pot > ground, maybe around 20K -- in case the pot fails; and so you don't bleed all bias voltage to ground if you turn the pot too low.

[bruno]

I'm measuring the bias current with a weber bias rite.

The bias current value is the same even with the original 39k in place. Which still measures 39k by the way.

Plate Voltage is close to 459v

Thanks

[phsyconoodler]

You need to inspect the bias supply components and measure the bias 'voltage' at pin 5 of each power tube.You likely will have to repair the bias supply as you probably have a bad diode or capacitor.
  When you fix it,measure for the maximum negative bias voltage you can get before you pop the tubes back in.Look for at least -50v at pin 5.

[jjasilli]

Sorry, I call that cathode current (not bias current).  But yes, do what psycho said.

[sluckey]

What you did will not affect the bias voltage very much because you don't have a voltage divider set up. Take a look at the typical Marshall bias circuit on page 5 of this pdf...

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

Make your circuit after that circuit and it will work. The 15K resistor is very important to making the bias voltage adjustable. It's not the value but the placement in the voltage divider that makes it important.

[bruno]

Thanks guys, a voltage divider does really make more sense, I don't understand why selmer used this type of bias circuitry.

[bruno]

Yesterday I returned to this amp... to finish the bias circuit! Upon power up, I find I have 460 volts on the plates, the cathode goes to ground, and around -157 volts on the grids that are fed by the bias circuit after the 220k resistors. I have no sound and I am not able to measure any bias current if I use a weber bias probe. I'm a little lost since everything seems ok.

I switched the bias cap and the diode for a 1n4007, which appears to do it's part fine. I don't understand why I get not bias current reading and no sound.

[sluckey]

and around -157 volts on the grids

That's why there's no cathode current. The tubes are way into cutoff. Lower that bias voltage a lot.

[bruno]

thanks sluckey!

[PRR]

> The values in the amp are really similar to

How "similar" are the bias-supply values??

> I installed a pot to regulate the bias

You need to get the voltage at the junction of the bias-cap and two 220K resistors to be pretty near negative 50 volts. Not 20 volts. Not 150 volts. Around 50V. Do this with the power tubes OUT.

Don't think "voltage divider". Second, it won't work with this very high impedance bias supply. First, when the pot wiper loses contact, bias is lost, amp burns.

What you want to do is confirm the 180K||2Meg resistors, then diddle the 39K to 35K or 50K. Surely no less than 30K. Therefore 27K or 33K fixed in series with about 25K variable rheostat.

[Ferrochrome]

> The values in the amp are really similar to

How "similar" are the bias-supply values??

> I installed a pot to regulate the bias

You need to get the voltage at the junction of the bias-cap and two 220K resistors to be pretty near negative 50 volts. Not 20 volts. Not 150 volts. Around 50V. Do this with the power tubes OUT.

Don't think "voltage divider". Second, it won't work with this very high impedance bias supply. First, when the pot wiper loses contact, bias is lost, amp burns.

What you want to do is confirm the 180K||2Meg resistors, then diddle the 39K to 35K or 50K. Surely no less than 30K. Therefore 27K or 33K fixed in series with about 25K variable rheostat.

Hello and happy easter to everyone! First post from me on this forum, sorry for resurrecting an old thread, but there never was a confirmation from the thread starter that the problem ever was fixed, and I'd really like to know if it was repaired with the 33k+rheostat adjustable bias mod.

I have the same problem with my newly acquired  Selmer TnB mkIII and would appreciate some help if anyone would be so kind.

The amp apparantly is all original except from replaced the three cathode bias electrolytics, new bias capacitor and a 20k trimpot in series with a 33k resistor, instead of the original 39k in the bias circuit. I added the trimpot because the current was skyrocketing, measured with bias probes. Also lots of loud irregular pops and crackling was coming from the connected speaker.

After confirming all resistors and diode in the bias circuit OK, replacing the bias cap, adding the trimpot and adjusting to max resistance of 53kohm, pin 5 on the el34's are measuring -40v with tubes out. (Before repair I measured -20v). Plate voltage is 460v

Plugged the old RFT el34's back in and 10 seconds after power up the current has climbed to approx 30mA. I have sound from the speaker (maybe a bit low volume) and no pops/crackling. Then suddenly the needle jumps up to ammeter max of 50mA and massive distorted noise is comming from the speaker.

So the question is, can -40v instead of -50v on pin 5 be the reason for this drastic jump in current? I would guess no, so what can cause it? I have no way of testing the el34's, but guessing one or both could be faulty. (I've checked all the ecc83's = OK). So before ordering a new pair of expensive el34's, does this appear to be a problem with the tubes or with the bias circuit?

There is nothing loose inside the el34's, and visually they seem OK. I hear no noise or microphonics when tapping the tubes.

[pdf64]

Is the HT voltage 460V with the EL34 fitted and drawing current, or does it measure that with them not fitted?
Do the EL34 you’ve got work ok in another amp? Or have they ever worked ok in a guitar amp?
If you can’t answer that, then it would be best to buy some known good valves, that have been run in at an appropriately high HT voltage.
A valve amp user needs spare valves anyway.

[Ferrochrome]

Is the HT voltage 460V with the EL34 fitted and drawing current, or does it measure that with them not fitted?
Do the EL34 you’ve got work ok in another amp? Or have they ever worked ok in a guitar amp?
If you can’t answer that, then it would be best to buy some known good valves, that have been run in at an appropriately high HT voltage.
A valve amp user needs spare valves anyway.

Thanks pdf64!

Found out I made a silly mistake, and that's why the current went to the roof. I had the bias probes in circuit while having a guitar plugged in and the volume turned on.

Detached the probes and the amp is now sounding great through the 18" greenback in my Selmer Goliath cab. The Siemens branded RFT tubes are still going strong, biased at 32mA on the hottest tube. Above that there was red plating, on the tube with the highest current, so not sure how long it will work. Voltage measured with the tubes in is around 420v.

So the new bias electrolytic cap, added pot in series with a 33k resistor for bias adjustment, tightening and cleaning of the tube sockets and the voltage selector on top of the power transformer did the trick in my case. This amp sounds great!

[pdf64]

Great!
Just to note that if the HT fuse blows, the magnitude of the voltage on the bias supply cap will increase. So make sure that the replacement cap used can cope with that.