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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 14, 2014 2:45 pm
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Aspiring Musician
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I did some research and understand that a choke is used like a resistor, blocking AC and allowing DC. I haven't researched Henries yet, but will in time. Is it possible to use a resistor in place of the choke as a test?

I read the choke lifted and it was 98.? ohms across.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 14, 2014 6:39 pm
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I think I've found another problem. I have grid voltage on V3 (.037vdc) and V6 (67.8 & 61.7vdc). V3 is minimal, but the DC voltage on V6, pins 2 and 7 isn't supposed to be there, is it?

I've got the .1/600v OD's and SoZo Mustards, but no .001's.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 14, 2014 7:22 pm
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You should be getting negative DC on those grids of the 6L6GC. From the bias supply. Usually around -50 to -60VDC. You definitely should NOT be getting positive DC. If you are, lift one leg of each coupling cap from the phase inverter and re-measure the VDC. If the voltage now read -50 to -60 VDC, replace the coupling caps.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 14, 2014 7:51 pm
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Thanks for the info, I'll do that tomorrow. I just hated the thought of having to replace any of the blue coupling caps from this amp, but they're worthless bad. I'm learning, even in my 50's I can surprisingly still retain it. 35 years of welding fumes didn't get all the cells.

Thanks for the willingness on this forum to help out. I know guys like me can be a pain in the "arse" sometimes.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Sat Mar 15, 2014 10:30 am
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You should be getting negative DC on those grids of the 6L6GC.

I've got negative DC on the 6L6 grids (pins 1 & 5 @ -44.6vdc), but I have +68vdc & +62vdc on the phase inverter AT7, pins 2 and 7.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Sat Mar 15, 2014 10:57 am
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Pin 1 isn't used in the 6L6GC. Pin 5 injects the bias voltage into the bottle. At -44.6 VDC, your pin 5 measurement seems a skosh low (should be around -52 VDC).

Arjay

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Post subject: Re: '64 Twin Reverb voltage question
Posted: Sat Mar 15, 2014 12:30 pm
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Retroverbial wrote:
Pin 1 isn't used in the 6L6GC. Pin 5 injects the bias voltage into the bottle. At -44.6 VDC, your pin 5 measurement seems a skosh low (should be around -52 VDC).

Arjay



Yes, sorry, 1 on the socket is used for connection/standoff points. The Fender schematic doesn't show there should be voltage on the phase inverter tube grids. I'm assuming it shouldn't be there. I'll start checking coupling caps.

I've learned now the main schematic is much easier to work with than the layout.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Sat Mar 15, 2014 3:12 pm
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A properly-drawn schematic is your BFF when troubleshooting.

Arjay

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Post subject: Re: '64 Twin Reverb voltage question
Posted: Sat Mar 15, 2014 4:13 pm
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Okay, it 's not good to have this much positive voltage (60+) on the grid as this will cause electrons to flow to the the grid as well as the plate, at least in a preamp stage. Does this stand true with the phase inverter as well? The 12AT7 grid should really read negative voltage shouldn't it?


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Sat Mar 15, 2014 7:27 pm
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You should have very little milliVolts of DC voltage on any signal tube grid (12AX7 or 12AT7). If you get more than +2VDC on any grid of these tube you will permanently damage those tubes. Signal grids are not designed to handle more than a few hundred mV DC.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Sat Mar 15, 2014 11:38 pm
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BMW2002Ti wrote:
You should have very little milliVolts of DC voltage on any signal tube grid (12AX7 or 12AT7). If you get more than +2VDC on any grid of these tube you will permanently damage those tubes. Signal grids are not designed to handle more than a few hundred mV DC.


+1

It's possible that you now have "crispy critters". You might drop those pre-amp tubes into a TV-7 or a Hickok and check them for damage.

Arjay

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Post subject: Re: '64 Twin Reverb voltage question
Posted: Sun Mar 16, 2014 6:21 am
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Retroverbial wrote:
BMW2002Ti wrote:
You should have very little milliVolts of DC voltage on any signal tube grid (12AX7 or 12AT7). If you get more than +2VDC on any grid of these tube you will permanently damage those tubes. Signal grids are not designed to handle more than a few hundred mV DC.


+1

It's possible that you now have "crispy critters". You might drop those pre-amp tubes into a TV-7 or a Hickok and check them for damage.

Arjay


I've got some new AT7's to swap. It's only the PI that has voltage issues. I've found 2 caps that are allowing dcv's to pass, so I'll change those today. The .1/200v that goes to V6 - pin 7, and the .047/400v that comes from V1 - pin 6. These must directly affect the grid voltages on V6.

Thanks again.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Sun Mar 16, 2014 7:25 am
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Yes, both caps effect the phase inverter. Pin 6 of V1 coupling cap loads the grid of the PI, when you are using NORMAL channel. Pin 7 of V6 coupling cap loads the lower half of the sine wave signal onto the PI. This coupling cap is directly connnected to the global feedback network.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Sun Mar 16, 2014 1:55 pm
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I did some research and it's been explained that PI grids can't be measured properly with DMM because of bootstrapping? (long tailed pair). There's supposed to be another way to measure it, across the resistors maybe, I need to do more research.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Sun Mar 16, 2014 2:45 pm
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upnorth wrote:
I did some research and it's been explained that PI grids can't be measured properly with DMM because of bootstrapping? (long tailed pair). There's supposed to be another way to measure it, across the resistors maybe, I need to do more research.


Which parameter can't be read at the PI grids? Fender has the pertinent voltages on their schematic. +98VDC (grid to ground point) and +100VDC (cathodes). With no signal input, you should have very little (like mV) of VAC (really should be zero VAC).


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