Help! Bias Confusion!

[shiner555]

Hi All,

Something's got me really stumped on my bench and I'm hoping some of you can help me out.

I'm working on an Earth G2000 guitar head, which is basically a fender twin rip off. However, I have completely changed the output section to a Marshall set up with EL34's. Brand new PT via MetroAmp. The bridge rec and entire bias circuit is brand new. Also brand new are the 5.6k grid stoppers, the 1k5Watt B+ resistors, and the 220k bias feed resistors.

For some reason, I can not get the voltages to match via my bias probe on V7 and V8, however the voltages measured on each pin of the tube socket while the amp is on, tubes are in, and idle are all correct. Really stumped. Here are the pin voltages for each tube, and also my bias measurments. Any help would be greatly appreciated!

Amp On, Tubes in, Idle
V7
1 0
2 3.3 VAC
3 495VDC
4 491VDC
5 -50VDC
6 491VDC
7 3.3VAC
8 0

V8
1 0
2 3.3 VAC
3 495VDC
4 491VDC
5 -50VDC
6 491VDC
7 3.3VAC
8 0

V9
1 0
2 3.3 VAC
3 495VDC
4 491VDC
5 -50VDC
6 491VDC
7 3.3VAC
8 0

V10
1 0
2 3.3 VAC
3 495VDC
4 491VDC
5 -50VDC
6 491VDC
7 3.3VAC
8 0

Via Weber Bias probe on
V7 Vp = 481VDC, Ik = 17.1
V8 Vp = 476VDC, Ik = 15.4
V9 Vp = 500VDC Ik = 24.6
V10 = 500VDC, Ik = 18.1

Where is the magic drop in Vp on V7 and V8 comin' from??

Thanks in advance!!

[Enzo]

Connect the bias meter to read plate voltage, and connect your DMM to the same plate. One would think they would both measure the same thing, being connected to the same point. if not, then ione instrument would seem to be out of calibration.

And another test, swap probe adaptors around and see if one adaptor measures different under the same tube and in the same socket as another adaptor.

[Amp Kat]

Those voltages look super and btw you should measure pins 2 & 7 across those terminals to get your 6.6 volts. Measuring from ground to each pin is measuring the CT to each side because the CT is grounded.

[pdf64]

Consider fitting 1 ohm cathode resistors inside the amp.
Then you can directly compare readings from your DMM and bias probe.
Do the tube sockets require re-tensioning?

[R.G.]

Where is the magic drop in Vp on V7 and V8 comin' from??

Just offhand, I'd say it's from the output transformer not having identical wiring resistances between the two halves of the primary.

The only difference in plate voltage that's possible at DC in a push-pull stage is the difference caused by (a) any difference in DC currents in the plates and (b) any difference in the DC resistances the current goes through outside the tube. The plate of a good pentode or beam power tube (6L6) is a pretty good current source, which is a way of saying that it pulls in a current, and the voltage there can be almost anything from 50V up to the full B+ and beyond. Different transformer resistances, different DC voltages.

What you're concerned with is not the plate voltage. You need to be sure you get the currents the same. The design of a push-pull output transformer is based on having the same DC current in both halves of the primary. Getting these to be nearly the same, within reason, is one of the major objectives of biasing. The voltage dropped across the transformer half-primary is way down the list in importance, as is the DC plate voltage of the tubes (again, within reason).

[shiner555]

Hi All,

Many thanks for the replies!! Here is a quick summary of responses.

@ENZO. I have moved the bias probes around and the low voltage issue totally stayes with V7 and V8 with the same tube. I am going to try and get some 1OHM resistors just to check that with a DMM vs. the bias probe, because there is a slight chance it is out of calibration. Although I hope not.

@RG Many thanks for the comments. That is exactly what I was thinking as well in-regards to the OT. I understand I'm not concearned with the DC voltages per-say, but when trying to bias...I don't quite know how to get the two sides to match with the voltage and or current draw differences? i.e. if I set V9 and V10 to say an Ik of 30ma, V7 and V8 will be way off becase the bias circut feeds both side equally. I have not been able to get the two sides even close.

Does this mean I need to get a new OT as well? Or do I have to add some kind of individual bias adjust?

[Amp Kat]

Hi All,

I understand I'm not concearned with the DC voltages per-say, but when trying to bias...I don't quite know how to get the two sides to match with the voltage and or current draw differences? i.e. if I set V9 and V10 to say an Ik of 30ma, V7 and V8 will be way off becase the bias circut feeds both side equally. I have not been able to get the two sides even close.

Does this mean I need to get a new OT as well? Or do I have to add some kind of individual bias adjust?

No it means you need to install dual bias pots so you can tweek each side accordingly to the bias point you need to match each side with. I personally wouldn't bother because I'm a distortion player and it may help me but if you play super clean and are a balance freak then by all means install dual pots.

[shiner555]

All,

It seems I've found and fixed the problem!! When I went to install the 1Ohm resistors on V7 and V8 it looks like the ground for those tubes really wasn't the proper ground. There was a jumper wire from V7 to V8 and then another jumper wire to the amp output connector ground. I hadn't noticed that before. So when I cleaned that up and put in the 1Ohms with a really short path to ground, the voltages for V7 and V8 now match up with V9 and V10!!! Also found my bias probe was correct.

Thanks to all for the suggestions here. I've learned a good lesson here....always make individual paths to ground from each cathode, and as short as possible.

Cheers!

[Amp Kat]

We all knew that , we were just testing you. lol !
Nice job.