erwin_ve wrote:Hi Claus,
The PI voltage is 331/327v which is too high as you mentioned.
Thanks!
That voltage is not nessarily too high.....are those voltages measured from pin1-to-ground respectively pin6-to-ground? I think they are.
You have to measure the from
pin1 to pin3 and
pin6 to pin8
Furthermore, you probably have around 50 volts to ground from pin6/8. If you deduct this amount from above mentioned voltages, you get around 281/277v - which is fine. (FWIW I have almost the same PI voltages in my mentioned amp)
erwin_ve wrote:Od and also Od with PAB are good sounding, no hum issues.
However the clean channel is humming like a bee(100Hz), so I suspect something is wrong at the Od/clean channel switching relays.
I'd suggest taking the relay out and hard wiring the clean channel to narrow down the problem. It sounds like you may have a grounding issue because it goes away with the OD side.
Also the voltages are high;
B1= 530v
B2= 529v
B3: 485v
B4: 401v
B5: 397v
F.e the plate voltage at v1a is 244V, v1b is 249v
Way too high.
My dropping string is 4,7k-22k- 2,2k
The Ps filtering at B1 is 2x100uF in series.
At B2 2x 68uF in series.
after that they are followed by the usual 1 cap(22uF) per node.
Anyone suggestions for my voltages and hum issue?
I remember similar voltages. I adjusted the dropping string, of course with tubes in, until I found a more reasonable point. I have the 150K resistor at the end of the power rail as well, which tends to help balance out the amps feel more than just adjust the voltages.
erwin_ve wrote:V1a is 203V
v1b is207v
v2a is209v
v2b is 214v
I think I'm in the ballpark; The od sounds much better!
For the Hum thing; I have a suspect: unshielded wire from the relay to the clean master.
Bob and Claus thnx for all the input; I really appriciated it!
Happy to help. I'd still drop those voltages a tad more. Get V1 and V2 below 200V seems to be where the magic happens, but this is just an observation, use your ears not your meter.
No but the other day I was re-working the way my bright cap was configured.
I used the layout that Tone from VVT Amps posted.
It turns the bright cap on in clean and disables it in OD.
When I tried the amp after that, the amp seemed unstable and if I tapped the chassis it almost wanted to oscillate.
Especially when I tapped on the pre-gain pot.
It was a WTF moment for sure. I didn't think anything I had done would cause this.
I have a vintage tube in V1 ( Raytheon long plate) so I knew that it was slightly microphonic but this was ridiculous.
Got out the chopstick and found the input jack was not shorting the input properly.
So I had to re-tension the switch part of it so it shorted the input to ground.
Solved the problem.
Not saying that's what is going on in your amp but something to check.
Make sure the cap is on the middle lug of the volume pot and the switch is to the CW lug.
Ceramics tend to be a bit noisy or microphonic but you might try either another cap or reversing the leads. Yep, I found one that was noisy one way but not the other way. Can't explain it.
Thnx for your response; For the record; this is a HRM. The bright cap I'm reffering to is the one on the clean master.
Anyway I tried several cer. caps.
Everytime I connect a cap(68pF) over the clean master I get a huge hum. Disconnected the hum is gone.
Every solder joint/ connection etc. is triple checked, swapped tube/ wires, swapped relays. In od mode there's no hum at all. So it's specific around the clean master volume area.
Hardwiring the clean channel; same story, cap removed= hum gone.
Yes I understand it is a HRM amp but what I was saying would apply to any of the amps with a bright cap on the pot.
And the bad jack could apply to any amp with a switching or shorting input jack.
Structo wrote:Yes I understand it is a HRM amp but what I was saying would apply to any of the amps with a bright cap on the pot.
And the bad jack could apply to any amp with a switching or shorting input jack.
Sorry if I came off a bit strong, I appriciate the comments and ideas!
Meanwhile: I decide waiting on a scope to arrive for further troubleshooting for the hum(only with cap on clean master)
Tweaked the dropping string; 8,2k for the 1st resistor. For v1 my voltage are 193/199. For v2; 199/ 205. It does feel and sound(overtones) better that way.(Thanks Bob!)
Adjusted the od trimmer to 25k to ground and adjusted the HRM tonestack for genuine d-tones
Bias; funny how a few mA does matter a lot for he od sound On B1;528v biascurrent: 32 mA for each 6L6GC tube sounds fuzzy, with 34mA it sounds nice liquid and creamy. Tommorow testing, friday first gig with it.
I will do my best to make a recording for posting it here.
Finally I figued out why it was humming.
The cer. cap on the clean master was picking up something in that area.
Attaching the the cer. cap to some wires with croc clamps it was't having this issue.
I soldered 2 wires attached to the pot with the cer. cap away from that hum position. The cap is mounted with some silicon in the area of the presence pot. No problems, quiet as it should be.
I'm very happy with it.
Pic with lay out; next thing is making a lexan cover for it.
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maker. When I remove the cap the hum is gone.
