The V1 guessing game

RVA

RVA

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If you did not have a schematic, or an instruction manual, and you had two 12AX7 preamp tybes, would you surmise that V1 was the one on the left or the right (viewed from the rear with the socket holes facing upward)? Or, is there another way to tell?
 
Thatbastarddon said:
Usually closest to the input jack....physically.
Click to expand...
That's what it seemed like. I swapped a 12ax7 for a 5751 tonight. I tried both slots and it was the one next to the input jack that made the difference. Of course, I am assuming that a V1 swap is what is required for this replacement to matter.
 
RVA said:
That's what it seemed like. I swapped a 12ax7 for a 5751 tonight. I tried both slots and it was the one next to the input jack that made the difference. Of course, I am assuming that a V1 swap is what is required for this replacement to matter.
Click to expand...

It totally took the "fuzziness" out of my gain. I am now running at 2 O'clock and clean, whereas before - with 12AX7 - I was at 10 O'clock and grainy.

You can feel the speaker movement!
 
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With T12AX7 in V1:

IMG_20180418_54647.jpg

With 5751 in V1:

IMG_20180418_52183.jpg

Look at the gain setting increase, but with less audible gain, more presence and more clean headroom....EQ's slightly changed for the new tube...
 
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Robert Herndon said:
With T12AX7 in V1:

View attachment 14096

With 5751 in V1:

View attachment 14097

Look at the gain setting increase, but with less audible gain, more presence and more clean headroom....EQ's slightly changed for the new tube...
Click to expand...
This would have been a great opportunity to provide sound clips of the differences. I know you can do it... I know you like to do it! Come on, Bobby... :fingersx:
 
syscokid said:
This would have been a great opportunity to provide sound clips of the differences. I know you can do it... I know you like to do it! Come on, Bobby... :fingersx:
Click to expand...

Ok, the Von Herndon Doubleneck was used in making both videos - Bridge pickup is a Thro-Bak SLE-101 - wood spacer, Bridge 8.1K, long A2 magnet

First video is with T12AXC7 Genelex Gold Lion in V1, and TAD 7025WA's in V2, V3, & V4. NOTE: In this old video, I switch the Attenuator on and off. Switch 'Up' is attenuated and switch 'Down' is bypass:




I just shot this video about 5 minutes ago, with the JJ5751 in V1:


 
Robert Herndon said:
Ok, the Von Herndon Doubleneck was used in making both videos - Bridge pickup is a Thro-Bak SLE-101 - wood spacer, Bridge 8.1K, long A2 magnet

First video is with T12AXC7 Genelex Gold Lion in V1, and TAD 7025WA's in V2, V3, & V4. NOTE: In this old video, I switch the Attenuator on and off. Switch 'Up' is attenuated and switch 'Down' is bypass:




I just shot this video about 5 minutes ago, with the JJ5751 in V1:


Click to expand...
Wow! That particular Marshall circuit is very sensitive to the differences of those two preamp tubes. On my moddified Carvin Vintage 16, the differences are more subtle but still very worth it with the 5751.
 
syscokid said:
Wow! That particular Marshall circuit is very sensitive to the differences of those two preamp tubes. On my moddified Carvin Vintage 16, the differences are more subtle but still very worth it with the 5751.
Click to expand...

Yes! I didn't expect that much difference. It's like I can go from clean to dirty now with just pick attack. before it was full-on grain all the time!!!
 
syscokid said:
Wow! That particular Marshall circuit is very sensitive to the differences of those two preamp tubes. On my moddified Carvin Vintage 16, the differences are more subtle but still very worth it with the 5751.
Click to expand...

Every Marshall I have owned in the last 30 years ended up with a 5751 in V1. I found out about this trick by accident - I was given a box of 10 NOS JAN-Spec RCA 5751's in the late '80's (they actually came from the Naval Research Lab in DC when they wholesday switced over from any tube-powered testing equipment to solid state. Someone who worked there just grabbed a box of tubes and gave them to me. Half were things that didn't go in guitar amps but the other half were usefule). When my 800 started getting wonky and I knew one of the tubes was going I ended up sticking one of the 5751's in and playing around with the position (can't rememver now what sequence I tried things in) and when I ended up with it in V1 my mind was blown. Transformed the amp. I've done the trick on all the master-volume amps that I have owned since, except for the Mesas which don't seem to benefit from it.
 
gball said:
Every Marshall I have owned in the last 30 years ended up with a 5751 in V1. I found out about this trick by accident - I was given a box of 10 NOS JAN-Spec RCA 5751's in the late '80's (they actually came from the Naval Research Lab in DC when they wholesday switced over from any tube-powered testing equipment to solid state. Someone who worked there just grabbed a box of tubes and gave them to me. Half were things that didn't go in guitar amps but the other half were usefule). When my 800 started getting wonky and I knew one of the tubes was going I ended up sticking one of the 5751's in and playing around with the position (can't rememver now what sequence I tried things in) and when I ended up with it in V1 my mind was blown. Transformed the amp. I've done the trick on all the master-volume amps that I have owned since, except for the Mesas which don't seem to benefit from it.
Click to expand...
How did you know the 5751, and other tubes you experimented with, were safe for the amp?
 
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RVA said:
How did you know the 5751, and other tubes you experimented with, were safe for the amp?
Click to expand...

I had/have one of these:

s-l225.jpg


(legend has it that this is the same book Leo Fender used to create the first Fender amps - there are all kinds of circuit diagrams in it)
 
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