Tube Swapping!

[DetroitBlues]

I watched a video the other day of how to alter the preamp volume to be more usable so the Fender Hot Rod Deluxe doesn't go from zero to 11 with a 1/10 of a turn.

By swapping the V1 12AX7 with a 12AT7, it lowered the gain a bit and made the preamp volume more usable.

Decided to try that on my Jet City 20HV last night.

Ended up pulling the entire chassis out of the amp to swap the tubes, which was a good idea considering I didn't know which preamp was the V1.

Lucky for me the printed circuit board was labeled.

For anyone who wants to know, it appears the 20HV is just the 20H with a couple mods including a separate circuit board for the effects loop.

Anyway, I decided to swap all the tubes with mid-gain tubes to try to clean this amp even more.

It worked.  All new tubes and the preamp volume needs to go to 11 to get really crunchy, but in a good way.  Turns this amp into a really flexible pedal platform.

The red circle is the preamp tube.... 

[TalismanRich]

Reverb.com just had a similar article about different preamp tubes.   There are quite a few lower gain tubes with compatible pinouts which can be used interchangeably.

PREAMP TUBE SWAPPING

 

[Millennium Maestro]

Thanks for sharing that!  I hope you have a wonderful first  weekend in August.

[tbonesullivan]

You usually want to be careful with using a 12AT7 to replace a 12AX7. While the voltage change is less, the overall power dissipation of a 12AT7 is far higher than a 12AX7. The bias current, load line, etc are spec'd differently. This can result in different plate voltages in the preamp chain, which can do bad things to the sound as well as the amplifier itself in some cases. There are better choices for a lower gain 12AX7 replacement, like a 5751 tube.

See this TGP thread: https://www.thegearpage.net/board/index.php?posts/23499521/

Anyway, tube rolling is nice, but you do need to be careful. Places like Doug's Tubes actually separate some preamp tubes based on the relative gain factor. Tung SOL makes a new production 5751. You can also find NOS versions for far less than NOS 12AX7s.

[RJLII]

What impact did it have on the high end? I tried a similar approach on a Blues Deville with an on/off switch for a volume control. It tamed the volume issues but I lost all the sparkle. I wound up changing it back. A volume pot with a different taper would have been a better move, but these were soldered to the PCB and not very practical for user servicing. 

[DetroitBlues]

The characteristics of this amp is close to a classic  Marshall amp, however this reminds me more of a Bassman now.  Regardless, it’s a great amp tone.  I really like it.

To your point, the pots are also PCB mounted so I couldn’t simply replace the pots. 

[davesultra]

With newer Marshall amp types I like to use a 5751 in either or the V1 & V2 positions. Helps make the DSL series amps much more manageable. 

[tbonesullivan]

Going back to this topic, I would recommend if possible that you put a Sovtek 12AX7LPS tube in the phase inverter slot by the power tubes. These are great tubes for warming up and an and getting a thicker sound. The stock GT tubes are usually Sovtek 12AX7WA tubes, which are very good in terms of being low noise, but not good in terms of good sound.

There are a lot of new production tubes coming out of the Chinese Shuguang factory under contract with Ruby Tubes and The Tube Store. There also are a lot of "reissues" coming out of the Russian "XO-Pul/Reflektor plant such as Tung-Sol, Mullard, etc.

Groove tubes are unfortunately often near the bottom of the barrel in terms of sound, but they are great for reliability.