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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Sat Sep 03, 2016 10:41 am
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Good carbon comp are the best. 1/2 watt is fine. 5% tolerance is preferred. Check the layout to see how the grid stoppers should be applied to the sockets.

See how you move the input line from pin 5, of the 6V6GT to pin 1? Then solder the 1500 ohm resistor across pins 1 and 5. The signal now goes to pin 1 (unused) through the grid stopper resistor to pin 5 (grid).

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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Sat Sep 03, 2016 11:15 am
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Went back and re-read your earlier post after the last question, took a bit to log back in. On the same page, re-direct input wire to pin 1 and solder 1.5k 1/2w resistor before dropping $150 bucks on PT. Sounds like good advice to me now that I'm on the same page.

Thanks for all the patience, have a good weekend.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Sep 07, 2016 4:08 pm
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Resistors will be here tomorrow, that was a good long weekend. Should be swapped out and be up ready by Friday. Keeping my fingers crossed.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Sep 07, 2016 5:33 pm
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Good luck! Hope it works.

:)


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Thu Sep 08, 2016 3:51 pm
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I ordered the wrong parts, got to rere-order.
Yes I'm having a $@!& day.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Thu Sep 08, 2016 4:34 pm
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I hate it when that happens.

:x

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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Fri Sep 09, 2016 7:06 am
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It was a $30 mistake on my part, but the delay is the harder part for me. I estimate the parts are from the early 80's and are beefy size, really looking forward to getting them back in the circuit.

I already got the two 1.5k series resistors on the 6v6's but went w/ pin 6 - pin 5 instead of pin 1 - pin 5, it was closer and didn't require me to remove a portion of the original ground straps. Hasn't been tested yet, waiting until I get the new order of carbon comp resistors.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Fri Sep 09, 2016 8:15 am
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sfceric64 wrote:
It was a $30 mistake on my part, but the delay is the harder part for me. I estimate the parts are from the early 80's and are beefy size, really looking forward to getting them back in the circuit.

I already got the two 1.5k series resistors on the 6v6's but went w/ pin 6 - pin 5 instead of pin 1 - pin 5, it was closer and didn't require me to remove a portion of the original ground straps. Hasn't been tested yet, waiting until I get the new order of carbon comp resistors.


Going from pins 6 (unused) to 5 is fine. In fact, I believe some early tweeds had this setup. The reason for grid stopper from pin 1 to 5 --- is to leave pin 6 open for a screen resistor (pins 4 to 6). Your amp doesn't need a screen resistor, as the 10K-ohm dropping resistor in the PSU acts to drop the voltage onto the screen (versus the B+ on the anode plates).

Hope all other fixes work! :)


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Mon Sep 12, 2016 10:34 am
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It's been three days since the last sitrep......que pasó?

Arjay

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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Mon Sep 12, 2016 2:23 pm
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I hear ya Arjay,
parts are coming via USPS so I expect them to be delivered and installed tomorrow sometime. I'll let you know as soon as I get it fired back up. Will include prescribed photos of course.
I'm glad I re-wired the OT, it is so much easier to remove the chassis w/ an extra bit of wire and have the cab close.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Mon Sep 12, 2016 3:24 pm
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Hokey dokey, standing by......

Arjay

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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Tue Sep 13, 2016 6:14 am
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sfceric64,

Good luck! I like to use this on circuits --- sitting around, ready to be fixed or completed. It's my little secret weapon. :lol:

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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Tue Sep 13, 2016 11:00 am
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Well the mailman showed up early today!!
The parts all tested w/in specs and were installed with some minor grunting and a minor singe of the ol' pointer finger. Damn that was hot, really tight fitting the standard 2w resistors back in place.
The plate voltages are still to high!! V1 is 97/101vdc(+5/+9), V2 is 182/183vdc(+32/+33), V3 is 368vdc(+88) and V4 is 367vdc(+87). The B+ is 373(+23).
Off to the transformer store. Here is what the final layout looks like.
Image
Image


Last edited by sfceric64 on Mon Jul 10, 2017 10:46 pm, edited 1 time in total.

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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Tue Sep 13, 2016 12:30 pm
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Very nice job! :D

What is the outlet voltage in your house? My bet it is above 120VDC. Maybe, close to 125VAC at times. The original PT in a 1950s era amp, is spec'ed for ~118VAC. Now, if the nominal secondary B+ voltage off 118VAC is 350VDC... a quick-&-dirty ratio of 2.966. Then, the voltage off say 125VAC would be around 370VDC.

That prolly is the problem. And a new PT will not solve it. Only a weaker rectifier, more resistance in the PSU rail, or modifying the PSU rail by adding a choke or some such major surgery --- will cure the high voltage issue.

Do you have a Variac? Try this experiment. Crank the voltage on the output of the Variac to 118VAC. Plug in amp. Now, measure those points again. My bet, they will be closer to nominal values.

There is another you can do. Nothing. This amp uses cathode tied resistors to set the bias of all the tubes. So, within a range there is a self-leveling of idle dissipation. If there is more voltage on the anode, the tube will draw less current --- to get to about the same watts of dissipation. This is why, you never need to reset the idle bias on any gain stage (12AX7, 12AT7, 12AU7, etc) tube, when replacing them with new tubes, different brands, or sub-types.

The 6V6GT output stage also uses a cathode-bias, cathode tied resistor to set the idle dissipation. No adjustments. As long as you are using healthy tubes (brands and types) noted for good tolerance to voltage --- you should be good to go. As long as you are not getting any red-plating in the 6V6GT... or getting early distortion in the volume range.

There is a fix that maybe not as painful (at least for the gain stages). Increasing the 10k-ohm dropping resistor in the PSU. If you go to say, 12k-ohm, 5 watt resistor --- this should drop voltages on the 6SC7 about 20% of what you are seeing now. But, I'd only do this if you are getting early onset of OD distortion, which you feel is due to excessive voltage on those gain stage tubes (6SC7).

You can also think about the 6SC7GTY mil spec tube. Both Raytheon and Tungsol made very good ones. This is why I went to the mil spec RCA 5693 (red rocket tube). It's a well-known, tough version of the 6SJ7. A tube used in many very early Fender amps.

Good luck! HTH! :)


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Tue Sep 13, 2016 12:59 pm
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Quote:
What is the outlet voltage in your house?
119-120, I check for the 1st 5 minutes of operation.
I don't have a variac.

Quote:
This amp uses cathode tied resistors to set the bias of all the tubes.
V1 is grid to ground biased, that's why it is 97vdc and is .004 on the cathode vs V2 is cathode biased @ 182vdc and 2.3vdc on the cathode.

Quote:
The 6V6GT output stage also uses a cathode-bias, cathode tied resistor to set the idle dissipation. No adjustments.
Its running at 12.8 watts at idle. It should be closer to 80% of max dissipation. Theres no room to SAG from there, while dissipation values adjust its one directional and sounds like dookie. Very little breakup when its dropping.

Quote:
You can also think about the 6SC7GTY mil spec tube.
That's what I'm using for V2. Its a better PI than the metal 6sc7, more stable.

While I believe a small increment change in the power dropping resistors will lower the voltages at its given stage, I have to go with my gut at this point. Incorrect high filament and plate voltages are a direct result of the power transformer being out of spec.....


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