Help please, too high voltages.. (voltages ok + sound)

mat · Post by **mat** » Tue May 15, 2007 7:26 am

I did my first fire up without the tubes and after some measuring I have way too huge plate voltages on the pre amp.

(without tubes)
B1 507
B2 506.8 (after choke)
B3a 500 (after 54k) - from B2 ?????
B3b 496 (after 57k) - from B3a ?????
B4 496 (after 2k2)
B5 417 (after 22k)
B6 412 (after 2k2)

Plate resistor are as on the layout: http://bad-domain/97HRM/97_ODS_HRM_Loop.JPG

V1 403,406
V2 412,413
V3 498,496 (d-lator (B6))
V4 492,491

V5 507 to OT secondary.
V6 507 to OT secondary.

The topic + pics of my build:
https://tubeamparchive.com/viewtopic.php?t=2277

I suppose the fact that the tubes are not installed could cause such a high voltages ? Should there be a resistors from the 22uF d-lator filter caps + terminals to ground like on the other filter caps ?

All groundpoints seems to be fine. Bias adjusts nicely and I get 3.5VAC to ground on both sides of the heater pins.

I'm little unsure to put the tubes in for such a high voltages..

dogears · Post by **dogears** » Tue May 15, 2007 12:09 pm

Put the tubes in...

mat · Post by **mat** » Tue May 15, 2007 1:31 pm

dogears wrote:Put the tubes in...

ok....shhh....

LOL

Bob-I · Post by **Bob-I** » Tue May 15, 2007 9:31 pm

They sound about right for no tubes.

Don't forget to measure at the plates and cathodes of the preamp tubes.

mat · Post by **mat** » Wed May 16, 2007 9:44 am

Bob-I wrote:They sound about right for no tubes.

Don't forget to measure at the plates and cathodes of the preamp tubes.

Ok,the voltages settled down with the tubes in (always learning) being:

B1 480
B2 479
B3a 339
B3b hmm.. did not measure that one..
B4 460
B5 331
B6 325

V1 1=200 3=1.85 6=207 8=1.72
V2 1=205 3=1.78 6=212 8=1.79
V3 (d-lator) 1=264 3=9.3 6=228 or 258 8=1.63
V4 (PI) 1=328 3&8=60.4 6=314

V5 3=477 4=477 5=-40 8=0.04
V6 3=477 4=478 5=-40 8=0.04

I have pair of JJ6V6's in it and I think I should bias them around 20-25mV. The bias was somewhere between 30-40mV.

The measurement was done by guitar unplugged and all but tone+presence at 0.

I get only constant hum (http://media.putfile.com/hum-82) out of the amp with guitar plugged (or unplugged) and volumes opened a little (or at zero). On 10s I switch the standby off and on.

The d-lator was bypassed with the switch.

Any suggestion where to start finding the bug ?

Thanks,

mat · Post by **mat** » Wed May 16, 2007 1:21 pm

Got it ! No ground wire from speaker jacks to the chassis. Man this thing is quiet, no hum no additional noise. Just the guitar itself. I did just a very brief playing test and both channels worked nicely. I did bias the 6V6S's to 22mA. I will report later how it turned up

Any comments about the voltages ?

dogears · Post by **dogears** » Wed May 16, 2007 3:02 pm

The Dlator cathode is really low.... You should have at least 15V on there. You could rebias it. Try 24k/1.8k for instance.....

Also, I'd shoot for 290-300V in pin 1.

mat · Post by **mat** » Wed May 16, 2007 3:49 pm

dogears wrote:The Dlator cathode is really low.... You should have at least 15V on there. You could rebias it. Try 24k/1.8k for instance.....

Also, I'd shoot for 290-300V in pin 1.

Thanks dogears, will try them tomorrow

mat · Post by **mat** » Wed May 16, 2007 7:10 pm

I suppose You mean 1k8 (in place of the 1k5) from the pin3 to the junction of 10k/1M and replace the 10k with 24k one ?

I'll try also another tube in there (nowSwetlana 12AX7).

I see on another schem 120V on the pin6. Is that what I should been shooting for ?

Thanks,

mat · Post by **mat** » Thu May 17, 2007 9:42 am

Ok, here is where I'm now:

I changed the d-lator tube to JJecc81.

Dropping resistors from B2 to B3a and B3b: 15K,57K

V3 (d-lator) 1=283 2=16 3=27 6=218 8=1.63

Otherwise the amp works fine but the hrm tone-trimmers seems to do nothing and when the d-lator is engaged it sounds very dark, lacking all the high frequences. Also one of the send or return pot works backwards also.

The fet-input works nicely as does all controls. The presence works backwards. I'll change the wiring on the pot.

I hope to nail the last bugs but eaven now the amp sounds gorgeous

Thanks to Normster for the layout + dogears for the tweaks + help.

dogears · Post by **dogears** » Thu May 17, 2007 12:12 pm

mat,

Use a 12AX7 in the loop. And yes, 1.8k and 24k. You identified the correct resistors. The amp should not be so dark with the loop on.

mat · Post by **mat** » Thu May 17, 2007 1:11 pm

dogears wrote:mat,

Use a 12AX7 in the loop. And yes, 1.8k and 24k. You identified the correct resistors. The amp should not be so dark with the loop on.

Ok I'll put a fresh 12ax7 in there.

The pin 3 (cathode) is now 27V, would that be ok ?

edit- with 12ax7 pin1=332 3=20 6=275

I nailed the hrm-bug. The wipers of the 50k (47k) and 1M were not connected to other side of the pot. Now the trimmers work ok

This is how it sounds with the d-lator engaged (at 23 seconds) : http://media.putfile.com/latorbug.

It is wired as in Normsters layout :

[img

dogears · Post by **dogears** » Thu May 17, 2007 4:49 pm

Great!

Reduce the pin 6 plate V to 220V now.... 275V is way too much. I'd reduce the supply to pin 1 too.... 300V on pin 1 and 220V on pin 6 is about right for me.

Try raising your 22K dropping resistor to a 47K. Then raise the 2.2K to B3B to something a bit bigger.

I think I have 47k/47K on mine.

Also, raise the 1K feeding B4 to a 2.2K

mat · Post by **mat** » Thu May 17, 2007 4:57 pm

dogears wrote:Great!

Reduce the pin 6 plate V to 220V now.... 275V is way too much. I'd reduce the supply to pin 1 too.... 300V on pin 1 and 220V on pin 6 is about right for me.

Try raising your 22K dropping resistor to a 47K. Then raise the 2.2K to B3B to something a bit bigger.

I think I have 47k/47K on mine.

Ok, back to swapping resistors

Thanks,

dogears · Post by **dogears** » Thu May 17, 2007 5:00 pm

Check your PM please!

mat wrote:
dogears wrote:Great!

Reduce the pin 6 plate V to 220V now.... 275V is way too much. I'd reduce the supply to pin 1 too.... 300V on pin 1 and 220V on pin 6 is about right for me.

Try raising your 22K dropping resistor to a 47K. Then raise the 2.2K to B3B to something a bit bigger.

I think I have 47k/47K on mine.
Ok, back to swapping resistors

Thanks,

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