You want to trade something for it like a few sockets?
I found this one article that converted a Hammond to a guitar amp.
Here is a snippet:
The AC power input circuitry was modified from the original Hammond circuit. The power transformer is old enough that it was designed to run on 115V mains instead of the 120V mains found today. Running the stock amp on 120V produces higher filament and B+ voltages than desirable. This problem can be easily fixed by putting the 5V rectifier filament winding in series with the AC primary winding. The 5V phasing must be correct, the easy way to test this is to try both orientations and monitor the 6.3V filament winding, use the lower wiring that produces the lower voltage. When the tubes are connected, the filament voltage is very close to 6.3V.
So what do you think of that?
In that conversion they used solid state rectifiers.
I wanted to use the 5U4 if I could. How much is that going to bump up the heater voltage if I use the filament windings as is using a 120vac supply?
If I lift the filament center tap with higher resistors like 200R would that do anything to the heater voltage?
I've found that those AO-29s in the average guitar amp circuit will put about 6.8VAC or so on the filaments. That's large for sure. No, 200R or even 2000R won't make any difference. I have put .1 ohm 5 watt resistors in series with the filament leads with some success. Just figure out all the filament loads and how much voltage gets dropped by each tube then divide your load by the full voltage to get voltage per ohm right? R=E/I right?
The other method is a really good one. It's called a buck/boost or particularly in this case a buck connection. I wrote an article that is way back there someplace in my 1500+ posts. It explains the theory and practicum of the deal. Cat's whiskers really. I'll pay postage for your un-needed expression control and e-mail you the file on how to do the buck program. How's that?
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
zeners don't work in this case because of the diode voltage loss is over a volt. More like a volt and a half, so you end up with something in the low 5 volt range. If you use low loss shotky diodes in a bridge with a cap entrance you may get enough to use a simple zener regulator or better yet a regulator chip. But, the buck program really is the best way to go, and it will drop the HT voltage a bit too. With a SS rectifier these PT's really go to town. Way more voltage than I would subject a 6V6 to. Your OT is designed for a pair of fixed bias 6V6s running at 320V on the plates and about 11 watts out. -19Vdc on the grids.
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Honestly, I wouldn't really stress 6.8VAC on the heaters, that is within 10% and unless you have a really good meter, there's no point in trying to get more accurate than what you can measure.
FWIW, I was working on a Carvin the other day. Unloaded, it's got about 7 volts on the heaters, but as you plug in the tubes one by one, the heater voltage drops down to four point eight volts. I couldn't believe it. The amp is about 15 years old, heavily abused, all original Russian tubes, only one 12AX7 out of its 9 tube compliment tested below spec.
Life is a tale told by an idiot -- full of sound and fury, signifying nothing.
Sorry to disagree Alexo but tubes were designed for 6.3 volts and 8K to 10K hours of operation. There is no debate in the literature that high filament voltages drastically reduce tube life.
Congratulations on having a Carvin that still works.
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
skyboltone wrote:
Congratulations on having a Carvin that still works.
I'm as surprised as you.
I'll admit I haven't done life tests comparing heater voltages, and I certainly won't argue for a position I can't defend, but many datasheets spec a 10% tolerance on filament voltages.
EDIT: I may be wrong about this. It seems that the later versions of many tubes with A suffixes are spec'd this way, while earlier ones are not. E.g. the 6V6GTA's states that "Except for heater ratings, the 6V6GTA is identical to the 6V6GT," and the only difference there is that the A is rated for 6.3 +/- .6. But that datasheet also calls the 6V6 a Pentode.
And you never know when that amp will be plugged into a socket that puts out 125VAC, or more... so you might as well play it safe.
Life is a tale told by an idiot -- full of sound and fury, signifying nothing.
I'm going to cut down the Hammond chassis into a Deluxe size.
I'll cut it shorter then bend up the tabs with some duck bill pliers and then pop rivet the corners. Should work.
I just need to figure out the chassis size I want.
I will be making the head box for this so I can make it anything I want but will try to make it the same as a deluxe.
Pretty sure I'll make an amp head out of it, sure a combo would be handier but I'm running out of room in my music room so a head could be stacked anywhere.
Yep, the Hammond metal put up a tough fight but it bent under my will!
Too bad the transformer is at the wrong end of the chassis for a normal build. I'll have to make the main board backwards.
And I thought this was going to be easy...................
I had a couple of face plates made in exact reverse of what I had already built an amp. I had already drawn the board in autocad, so it was relatively easy to just mirror image the drawing and rebuild the board as needed.
I always draw out my builds.
It would work in something as easy as Microsoft Paint.
I prefer Autocad because I have it, and I use it frequently.
I had a couple of face plates made in exact reverse of what I had built on the board.
I'd drawn the board in autocad, so it was relatively easy to just mirror image the drawing and rebuild it as needed.
I always draw out my builds.
It would work in something as easy as Microsoft Paint.
I prefer Autocad because I have it, and I use it frequently.
Would adding a choke input filter equivalent to the field coil drop the voltage and stiffen the supply enough to use 6l6 tubes or just drop voltage enough to use 6V6's without sounding harsh ? If so what value would you suggest SBT ? Does anyone know the inductance or dcr of the FC speaker ?
