Tweedle Dee, strange voltage...

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MHProd
Posts: 167
Joined: Tue May 07, 2013 6:47 pm

Tweedle Dee, strange voltage...

Post by MHProd »

Built a TD which sounds way too dark. Checked component values several times, ground continuity, etc...

Everything is pretty much stock except for Channel 2 has Marshall values (2K7/.68uF and .0047uF). Voltages is good everywhere (400V on the plates, 362V screens, 311V PI and 282V preamp).

V1 plates are 160V pin 1 and 114V pin 6. I was expecting something around 175V (it's a 12AY7, should it be 12AX7??).

PI voltages:
1. 206V
3. 1.55V
6. 257V
7. 20V
8. 47V

The amp sounds like the tone control on my guitar is down 5! With the tone control maxed out, it's still dark.

Any idea?
jph118
Posts: 53
Joined: Wed Apr 19, 2017 1:35 pm

Re: Tweedle Dee, strange voltage...

Post by jph118 »

Had a similar problem recently on a Princeton rebuild, turned out that the input jack was wired differently than the switchcraft J12a jacks I usually use... a ton of top was being sent to ground off the input. Pulled my hair out for a few days before I realized it!
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drew
Posts: 720
Joined: Wed Jun 21, 2006 2:49 am

Re: Tweedle Dee, strange voltage...

Post by drew »

MHProd wrote: Wed Apr 26, 2017 6:23 am
V1 plates are 160V pin 1 and 114V pin 6. I was expecting something around 175V (it's a 12AY7, should it be 12AX7??).
All preamp tubes are 12AX7, according to Charlie Wilson: http://ampgarage.com/forum/viewtopic.ph ... 15#p304284
MHProd
Posts: 167
Joined: Tue May 07, 2013 6:47 pm

Re: Tweedle Dee, strange voltage...

Post by MHProd »

I checked the input wiring and it's all good. Probably the one thing I didn't think about. I'm really baffled about the lack of top end...
lovetone
Posts: 297
Joined: Thu Mar 20, 2008 6:10 pm

Re: Tweedle Dee, strange voltage...

Post by lovetone »

Warning please check all of the below with caution valve / tube amps can Kill.

Can you measure the volt drop across the 10K dropper resistor feeding V1? with ohms law we can calculate the current draw this will give an idea what is going on.
and then...
The first thing I would look at with this are the strange voltages around the second triode pins 7 and 8

It may be that it is oscillating due to the lead dress. Disconnect the anode (pin 6) from the 100K on the board, then run the amp and check the voltage on pin 1 If this has risen significantly then this might indicate that the triode is pulling current I would think due to oscillation. Also check the voltage on pin 7 if it still has volts present that this might be leaking through the board material?

At this point you can try your guitar through the first input to check for any difference in tone?

I'll check back in a while
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