vintage Ampeg SVT - 12AU7 in place of 12BH7

[boroman]

I'm wondering if I could use 12AU7s in place of 12BH7 (usually microphonic and hard to find or low quality current production) in a vintage (1969-1980s) SVT amplifier? They're pretty similar tubes - in my opinion the ffect could be cleaner signal (will not push power section hard as BH7s). So, no problem with trying them in that place?

Also, would like to ask about phase inverter (12DW7). Ampeg in their service bulletin in the late 70s wrote that it could be replaced by 12AX7 with no circuit changes. But as I see from schematic, it uses both halves so it will not be the same (???).

[Helmholtz]

Also, would like to ask about phase inverter (12DW7). Ampeg in their service bulletin in the late 70s wrote that it could be replaced by 12AX7 with no circuit changes. But as I see from schematic, it uses both halves so it will not be the same (???).

Both the 12DW7 and the 12AX7 are double-triodes.

[Jon Snell]

A 12BH7 has a much larger anode dissipation than a 12AU7 plus the overall characteristics are slightly different.
It will work for a while.

[Helmholtz]

I estimate dissipation in circuit to around 0.6W per triode, so the dissipation limit shouldn't be a problem.

[boroman]

Both the 12DW7 and the 12AX7 are double-triodes.

It's 99,99% not just a "triode" that you can swap physically there
If 12DW7 is one half 12AU7 and second half 12AX7, and when both halves are used in the circuit, that will change the sound, right? So ampeg probably did not meant that. Correct me if I'm wrong.

[boroman]

A 12BH7 has a much larger anode dissipation than a 12AU7 plus the overall characteristics are slightly different.
It will work for a while.

What does it mean "it will work for a while". Does it mean the amp will burn, or will "eat" them faster? Do you know what will be sound upgrade/downgrade with these?

[Helmholtz]

It's 99,99% not just a "triode" that you can swap physically there
If 12DW7 is one half 12AU7 and second half 12AX7, and when both halves are used in the circuit, that will change the sound, right? So ampeg probably did not meant that. Correct me if I'm wrong.

If Amped states that you can use a 12AX7, I would expect that this works without changing the wiring.
About a sound difference you will have to find out yourself.

Can you post the original Ampeg statement?

[nevetslab]

Ampeg used the higher gain section in the front end, pins 6-7-8 (like a 12AX7 character), while the next stage is more like a 12AU7 Pins 1, 2 & 3, which has higher dissipation rating on the 12DW7. I've never tried this, and I don't see one in our rental inventory (CenterStaging, LLC Burbank, CA). I think I've only worked on one all these years. Just checked my service records.... I don't see one listed, so I was mistaken there.

[g1]

The original bulletin can be seen on pg.9 of service manual attached. It is clear that it is not a direct replacement in every position, but appears to be direct swap for power amp V1.

As far as changing the 12BH7 for 12AU7, and questions about 'will work for awhile', it's helpful to think about why Ampeg used driver tubes in the first place, and why not use a 12AU7 when it was a common Ampeg tube at the time? Driving that many output tubes takes some work. If 12AU7's were up to the task Ampeg would have used them. 12BH7 can do the job. Another example would be the Fender PS400, where they use a 6L6 power tube and drive transformer to drive the 6 power tubes.

edit: it is also important to note that the 12BH7 is being used to set the bias, and that could complicate using a sub.
Modern SVT variants use 12AU7 drivers, and they are much more prone to failure than the 12BH7's were. (yes, they 'eat' them)

[nevetslab]

While i agree with the differences and more drive capability of the 12BH7, I've found with the large collection of SVT-CL's and SVT-VR's in our rental inventory, I have rarely had to replace any of the 12AU7's. I did have more glass bottle failures in the early period of my servicing the gear before I had found success with the hi temp fitted siliocon rubber sleeving to shock-mount the KT88's and 6550 tubes in the chassis. don't know if that helped the driver tubes? I do know I got more life and much less damage of output tubes, but wold still have to cycle them out when they finally drifted out of balance,or outright failed. I just don't recall finding many failed driver tubes.

[g1]

My experience was primarily with the old versions running 12BH7's which rarely failed. Issues with 12AU7's is what I picked up from anecdotal information, where they are often called out as one of the first things to check, and especially when dealing with the power tube fault lights.
I'm glad to hear first hand from nevetslab that it is not necessarily the case.

Looking at how they do the new versions, it does seem that most values in the 12AU7 version are unchanged.

[x-pro]

Wouldn't a 5814 bulb work? Surely it's more reliable?

[boroman]

Wouldn't a 5814 bulb work? Surely it's more reliable?

I'd need to dig this more - never heard of this tube!

[Pedro Vecino]

The 5814 is a version of the 12AU7, probably better suited for computer use, but the 12BH7 is a different tube. The ECC99 JJ is closer but it is better to use an American 12BH7.

[boroman]

So, to sum it all because topic went more toward svt-cl side (?)

- what will be the difference between 12Ax7 and (original) 12DW7 on power amp in a vintage SVT?
- what will be difference between 12AU7 in place of 12BH7 as a driver in vintage svt?