Tube Amps / Music Electronics
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|6/11/2004 2:25 AM|
||6L6 Deluxe suggestions|
I have been lurking on this site to learn a few things and build my second amp. I built a deluxe amp using the iron from a crappy knight stereo amp built using 6V6s (2 per channel PP). Being a cheapskate I pulled everything from the old amp (recycling is good)and built it using a Hammond aluminum chassis but did use a weber fiberboard and mostly new caps and resistors. The hitch was that the amp was built for 4 6V6 tubes and used an EZ81 rectifier. I essentially built the amp to the deluxe circuit using the EZ81 and it sounds quite nice.
The problem, which I must admit I did not anticipate is the use of the oversized PT. With 6V6 tubes I get a cathode bias of 22.1 V and a plate voltage of 387.5-22.1=365.4 V. I am using a 258 ohm resistor which gives about 85 mA, very hot. I tried 6L6 tubes and got a cathode voltage of 26.2 V and a plate voltage of 378.3-26.2 = 352.1 V. So 101 mA and I calculate 35 watts. Still hot but more reasonable.
I have decided that I like the way it sounds with 6L6s, so I am going to buy a new OT to match the wattage. In the mean time, what damage might I do to the amp by using the OT designed for 6V6 output? (I am not too worried about the OT itself as I have two).
I also have too much gain in the amp such that the volume controls are very sensitive. I was thinking about putting a resistor in the circuit between the second gain and the PI. I have already replaced the 0.1 uf caps with 0.022 uf caps which dropped some gain and made my les pauls sound less woofy. I was also want to drop the plate voltage using a zener mod to lower the power.
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|6/11/2004 5:30 AM|
|Bruce /Mission Amps
Sounds to me like you're doing fine, know exactly what you want and are doing well with it.
An OT that can handle four 6V6s in near class A can't be half bad with a pair of 6L6s class AB1.
So what was the question?
|6/11/2004 7:02 PM|
The amp was a stereo unit, so there were two OT. So I am running two 6L6 with a PT made to run two 6V6. My guess was the unit was about 15 wpc, so I am doubling the output. It sounds fine, but I am curious if I can do damage to the anything running it this way. I plan to buy a 35 watt PT (Fender super size)for replacement. Is running two 6L6 tubes at 101 mA OK?
I was also curious if putting a resistor (500K ?)in series before the input grid of the PI was a good way to provide a bit less sensitivity to the volume controls. I get super crunch way before reaching halfway around and it is way loud. I would be interested in other ways as well. Since I am using the stock preamp, I assume this is simply due to doubling the power of the amp?
I also wanted to thank the board for supplying me with the knowledge to pull this off. This is really fun stuff.
|6/11/2004 5:35 PM|
|Bruce /Mission Amps
What preamp tube are you using?
Use one with way less gain.
Put a 1M resistor on the wiper to ground of the volume pot.
|6/11/2004 7:18 PM|
You should be fine using the old 6V6 iron from the old amp. I'm not sure about it's quality, but if it were a decent quality stereo ststem, the OPT iron would be nice and big for BASS (not needed on a guitar amp). Hammond OPT's are speced down to low frequencies for hhigh fidelity stereo system usage, which is why when most people choose a hammond for guitar, they coose one that handles half the wattage they need (they're tough).
If you do need more of an output transformer, you could try to parallel the two ones that you have. connect both of the conecter taps of the promary (HV input) together, allong with both of the plate windings. then connect both of the speakers terminals together at the common ground and at the speaker out. if you have several taps for speaker impednaces, only connect the trannies together at that point. that will double the power of what a single tranny could handle, and the impedance ratio of the new hybrid unit will be the same as a single one (try a thought experiment, either with parallel transformers that convert voltage, or impedances in parallel). Note that if you have very little output, you connected something wrong and they transformers are out of phase and the signal is essentially shorted out.
as for the volume, try a lower gain tube like a 12AY7 (or anything that can fit the socket like 12AU7s, 12BH7's, or if you rig a switch onto it to rewire the heaters, you can run 6CG7s (6SN7 in a 9 pin form) and other odd tubes that you find find in a "mystery grab bag of tubes" ), or add some resistoris in it. Try some grid stoppers to avoid blocking distorion if it does that
|6/11/2004 8:53 PM|
I have a lot of mullards and Sylvania. Some are 6FQ7 (the same).
Is a direct replacement for the 12AX7???
Can i use them in a guitar amp?? (My Ax84p1??)
Sorry for the offtopic.
|6/12/2004 3:26 AM|
When you mean a grid stopper, is that putting resistor in the signal path before a grid?
I am going to try using the OT as is and see how it goes. The paired OT is an interesting idea, that would be unique indeed. I am going to think about that awhile.
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