What to do with a JCM900 100W Output Tranny

[gingertube]

I have an original JCM900 100W Output Tranny (Raa=2K).

I really don't need 100W so I was thinking that rather than a quad of EL34 I might try 8 off 6V6s or 8 off EL84 instead for 50 to 60 Watts.

(Aside: I have something like 50 or more 6V6G, the old ST "Coke Bottle Shape" from the Sydney AWV Valve factory and a similar number of the later, smaller 6V6GT mostly RCAs, in EL84 I only have JJs).

Is anyone aware of this being done before? Links?
Good Idea/Bad Idea?
Better to swap the tranny for something lower power?
Any other suggestions?

Should I just do 4 off EL34 and add powerscaling?

Thanks in advance,
Ian

[Cliff Schecht]

Once you put more than a few tubes in parallel, they start fighting each other. You can of course take precautions to prevent this but then you start sapping a bit of power away from each individual tube (usually in the form of some cathode resistance). Then you have to somehow allow for each tube to have an adjustable bias so you can make sure nothing is going to run away. Maybe you could pair the bias off into pairs (four bias pots for 8 tubes) and then you could get away with matched pairs.

It's one of those things you don't see because the end results is not worth the fuss. If we were looking for additional clean output power then the only way to get there is more voltage or more output tubes, but we're not necessarily looking for either. More output tubes means more output power before you clip, but also you change the drive requirements and frequency response of the phase inverter/PA and may knock out too much high end. Some sort of cathode follower or source follower (MOSFET based driver) may be in order to get the drive requirements without sapping bandwidth. There are some details to designing these circuits that aren't obvious upon first glance but if you're still interested, I'm sure we could help you figure out any issues you might have.

[M Fowler]

I would just leave the amp alone and turn the volume knob down or sell it and get want you want.

[matt h]

(deleted)

[sluckey]

I have an original JCM900 100W Output Tranny (Raa=2K).

I really don't need 100W...

You could just use two EL34s. Since the primary impedance will be doubled, you'll also need to double the secondary impedance. IE, 4Ω tap becomes 8Ω. 8Ω tap becomes 16Ω. Etc...

[Cliff Schecht]

I would just leave the amp alone and turn the volume knob down or sell it and get want you want.

Wrong thread? It's a transformer not an amp..

[overtone]

If I had that many coke bottles I would go for a quad of 6V6G and double the secondary for Raa=4k4 because to me a quad always seems to be more satisfying.
On the otherhand, I would sell it. Just don't enjoy those 900s. It would easily find a new home and one could get something groovier.

[M Fowler]

Oh I didn't read that right thought it was a JCM900 amp, sorry.

Get a different transformer.

[roberto]

If it's a Dagnall C3070, it sounds pretty good in high gain designs with EL34s, around 420v B+ and no choke. Never tried on 6v6 amps or EL84s, but IMO it has not the right mids for the EL84. It's a bit too much flat.

[Cliff Schecht]

Oh I didn't read that right thought it was a JCM900 amp, sorry.

Get a different transformer.

If he were to run mismatched impedances to get the primary impedance up, this would work great for a 4-banger. A bit excessive size-wize but that will help not hurt.

[cbass]

How about 6 6v6s ? I've never ran the coke bottle ones. I dont know what you hqve for a pt But I've experimented with running some black plate sylvainas at 480 volts on the plates andthe screens around 420 and they seemed to hold up fine.

I bet it will be nearly as loud the 100 wattr though

[Cliff Schecht]

How about 6 6v6s ? I've never ran the coke bottle ones. I dont know what you hqve for a pt But I've experimented with running some black plate sylvainas at 480 volts on the plates andthe screens around 420 and they seemed to hold up fine.

I bet it will be nearly as loud the 100 wattr though

I've had similar experiences, ancient 6V6's are super tough tubes. So are the 6L6's, especially any militarized or industrialized variant. As long as you adjust the bias accordingly they can handle a lot of plate voltage before they start sweating.

[gingertube]

Guys,
Thanks for your thoughts / advice.

I built an amp for friend Neale with a quad of those ST Shaped 6V6G, used a JTM45 Output Tranny with an Raa of 3K4 and a B+ of 345V ( This was basically a London Power "Standard" from TUT 5 with power scaling). It is stunning.

Those 6V6G are wonderful. For example, here is my HiFi Amp:

I've been convinced that 8 off output tubes is not a trivial exersize but would not be daunted by doing a 8 channel bias servo + source followers for it.

So I have reduced my possibilities to swapping the tranny for a JTM45 or similar 50w tranny and doing a quad of 6V6G.
OR
Wrapping this up with another project I've been contemplating / collecting parts for, going seriously retro and throwing a quad (or 6) of 807 at it and using a phase splitting / driver transformer (which I have on the shelf too).

Cheers,
Ian

[shane]

You could try using four EL34's but run it at a lot lower voltage say about 315V or so.

PRS' David grissom DG50 runs four cathode biased EL34's at about that to get 50W.

Nothing to stop you running 6V6's at that voltage either I guess.

Cheers
Shane