Converting Carmen Ghia to 4xEL84 output, a few questions.

[nateflanigan]

Hi, A while ago I built one of those botany bay Carmen Ghia clones with the hammond organ parts.  It's pretty cool, last night I was playing a strat through it and loved the combination.  It's a fairly loud amp for at home use, but it just doesn't cut it at band practice.  I'm thinking it might be more useful as a 30 watt amp.  The power transformer is huge, so I'm pretty confident it can handle the extra load, obviously I'll need to change out the OT.  I just have a few questions about the differences in the out put stage of the Ghia as is and the hoffman AC30.


1) I have a 150 ohm resistor and 25uf cap on the cathodes, the AC has 50r/220uf.  I'm not positive but I bet I've got to change this, what would maintain the tone I've got but accommodate the additional tubes?


2)The resistors going from the PI coupling caps to the grids, currently 2.7k, on the AC 1.5k, should these be changed?


3)The AC's power supply uses a choke, the ghia's a 500 ohm resistor.  Should this be changed?


Anything else I should take into account?


Here's the Botany Bay schematic, thanks!


https://www.dropbox.com/home/schematics%3Amanuals/SCHEMATICS/GUITAR%20AMPS/Dr%20Z/Carmen%20Ghia?preview=BotanyBay_v16.gif

[Willabe]

I clicked on the schemo link and it says need to log in.

[nateflanigan]

Egads, attached the old fashioned way.

[drew]

Are you talking about the AO35 Hammond?  If so, your assumption regarding the PT may not be safe.  I can't remember which of the amp forums I read this on, but basically a guy indicated that he had done some investigating on this particular PT and found that, notwithstanding how big it looks, it's not suited for more than what it's called upon to do in the AO35. 

[nateflanigan]

AO35 indeed, I'll have to try and find that.  Thanks for the heads up. 

[nateflanigan]

Does this ring a bell?
http://ampgarage.com/forum/viewtopic.php?t=7650&sid=8d33b00e94f690379f576a42e198af6d


I have murdered two AO-35s and will not kill another. I would sell it on The Organ Forum and let folks install it in a B3/A100 where it belongs. I sent both the PT and the OT to Mercury and he has the specs. The PT is waaaay soft compared to most guitar amps. The secondary HV taps measure 260 ohms or so end to end. Both Ao-35 OT that I measured were spot on 8K primary. That's what MM came up with too. That doesn't mean that your's doesn't measure 29 but I've found that Hammond Organ OTs intended for EL-84 range from 7200 to 8000 ohms. As has been stated this is not a suitable rocket platform. It was the starting point for the Ghia, Matchless Spitfire, and Goodsell super 17. I built one liverpool two tube clone and one sorta bastard brown deluxe with the two and both owners are happy but there are a million other ways to get there. An A0-29 is a far more plentiful and cheap and has a 10,500 primary laydown OT just like the AO-35 and is a larger chassis to boot; but once again the PT voltage is waay too high for a Rocket. In fact it's perfect for a Tweed deluxe as long as you stick with the 5U4 rectifier. The PT is 375-0-375 at about 140ma even though it's huge, it's pretty soft. I got probably three of those PT around. I think that all of the OT from that era will work really well for guitar. The 7200 primary ones are found on just about any EL-84 Hammond Chassis except the AO-35. I've used a ton of them. In fact Abstact has one on his mini express.


I called MM this afternoon, not exactly a clear answer, mostly they just wanted to sell me a transformer, but they said it should handle the extra current.  They didn't want to give me the specs though.  So, who can say.

I found this great post about testing a transformer's current handling...
http://forum.allaboutcircuits.com/threads/assessing-an-unknown-transformer.38273/


So, we'll see, seems like some more research is in order.  Assuming the PT is OK, any insights on the other questions?