Is it safe to run a 12BH7 in a JVM1H 1 watt 50th?

[jl2556]

Swapping the 12AU7 for a 12BH7 should increase the output to around 7 watts or so. Will the power transformer handle it?

 

[DonP]

I tried one for a short while (just testing) in my DSL1HR. The power transformer will handle it. Some specs I've read say the 12BH7 likes 1/2 the impedance that a 12AU7 likes. So I tried 8 ohms (but only 2 speaker instead of 4) as well.

It will not increase the power to 7 watts because you would need to increase the power supply voltage with a transformer swap, and likely get a new output transformer to allow more power to get to the speaker.

I couldn't make up my mind and just put the stock tubes back in. Curious to see how you like it.

 

[Brett336]

I have been running one in my JCM-1 combo and have had no problems. I notice a little more bass content and separation in the tone. The 12AU7 sounds and feels a little more compressed in comparison.

The 12BH7 specs says it draws more current for the heater. Looking at the specs on the amp schematic I decided that it could handle it. No problems after a year of occasionally playing with it for an hour or so each time.

Not to change the subject but the biggest improvement for that little combo amp was finding a NOS Marshall branded Celestion G10S-50 speaker. It really beefed up the bottom end the amp and that little combo cabinet.

 

[Dogs of Doom]

I've run 12BH7A in my JVM for years.

I think it's a big improvement. Makes the amp sound bigger, w/ more headroom. It actually comes to about a 2 watt amp, w/ the 12BH7.

You can not run a 12AX7 in the power slot though. It'll cut in/out like there's a short & the distortion will sound bad.

I also tried a 12AX7 in V3, but, IMO, it took away the JVM character. It seemed more high gain, but it could have been a DSL, or any other amp. IMO, adding a 12BH7 to that slot also makes a difference, but, when running a 12BH7A in both V3 & V5, (should have accurately said V1) it doesn't really give you any advantage. Try 1 or the other. I think the power slot is best, but, try both. You might think differently...

 

[junk notes]

Anticipating a delayed Mojotone 1w arrival, I picked up a vintage Amperex grey plate - long plate for the lone power tube. A Bugle Boy will go in V1 and still deciding the other preamp tube positions. I am not familiar with this tube so I will have to pick up a 12HB7 short plate to compare, which means another grey plate type.

 

[Biff Maloy]

I've run 12BH7A in my JVM for years.

I think it's a big improvement. Makes the amp sound bigger, w/ more headroom. It actually comes to about a 2 watt amp, w/ the 12BH7.

You can not run a 12AX7 in the power slot though. It'll cut in/out like there's a short & the distortion will sound bad.

I also tried a 12AX7 in V3, but, IMO, it took away the JVM character. It seemed more high gain, but it could have been a DSL, or any other amp. IMO, adding a 12BH7 to that slot also makes a difference, but, when running a 12BH7A in both V3 & V5, it doesn't really give you any advantage. Try 1 or the other. I think the power slot is best, but, try both. You might think differently...

I need to try some of your tube recommendations. I knew you had really explored yours. My JVM1 still has the stock tubes from 2012. It's the least played of all of my 50ths. It's one that i like at first when i get it out but i never stick with it. Just too much gain overall.

 

[Dogs of Doom]

I need to try some of your tube recommendations. I knew you had really explored yours. My JVM1 still has the stock tubes from 2012. It's the least played of all of my 50ths. It's one that i like at first when i get it out but i never stick with it. Just too much gain overall.

If it's too gainy, you might try a 5751 in V4...



note: for the suggestions above, use this to figure out where to put things. I had said V5 for the power tube, but, it's actually V1 on the board.

I used the board, here. The view is flipped horizontally.



I marked the V# next to where it's printed on the PC board...

 

[junk notes]

So V1 on a Marshall JVM1H 1 watt is the power tube. Interesting.
(I picked up this: https://i.ebayimg.com/images/g/rWkAAOSwrdVfSr4K/s-l1600.jpg)

 

[Dogs of Doom]

RCA/Amperex...

 

[Biff Maloy]

Thanks!! I do have a nice 5751 with no home.

With all the gain and compression it has I usually end up prefering my vintage spec Stratocaster with it which makes sense. A humbucker just ends up being too squished to me.

 

[Biff Maloy]

If it's too gainy, you might try a 5751 in V4...

View attachment 79358

note: for the suggestions above, use this to figure out where to put things. I had said V5 for the power tube, but, it's actually V1 on the board.

I used the board, here. The view is flipped horizontally.

View attachment 79359

I marked the V# next to where it's printed on the PC board...

4 years and 3 days later I finally dug out my JVM1 and ran some tubes through it. It'd been put up so long I had to clean the pots.

JAN 5751 V4, Gold Lion ECC83 V2 and V5 with a EH 12BH7 in the V1 power slot.
I plan to experiment more but just used what I had.

What a huge difference! The gain control is so much more usable now. On the 50 and 100 watt models there's a "Keep the gain low" approach but to me that thinned the tone a bit on the 1 watt. To run it up to thicken brought on too much compression and distortion. Now I can run it to around 11 o'clock and it's full sounding but very crunchy with a presence to it. It's also a bit louder with the 12BH7 plus those Gold Lions are very good at quieting background hiss. Amp is very quiet compared to before.

I'm running mine into a 2061CX with a Heritage G12M20 up top and a Heritage G12H 55hz down low. Boss RE 202 Space Echo in the loop, Whirlwind Orange Box and Crybaby out front. First time I've really liked this amp in 12 years. But, I have played the others in the 50th Collection extensively over the years.

Thanks for the heads up on the 12BH7.