I have a 6v6 version of an 18 watt Marshall with a long tail pair (Schmidt) phase inverter.
I read that the typical el84 design doesn't like to have a large signal on its grid (compared to the 6v6) and a suggestion was made to me to change the tail resistor so that the phase inverter would pass more signal on to the power amp. Is there any possible drawback to doing this?
It seems like (if this works) this tail resistor would be a great place to put a pot. Wouldn't it be an excellent way to alter the sound of an amp that relies primarily on the power tubes for distortion?
Bonus question.
The OT is for 6l6 tubes. It's about 4400 ohms w/ 4, 8, and 16 ohm speaker taps.
I'm plugging an 8 ohm speaker into the 4 ohm winding. That ought to be 8800. How risky would it be to try running the 6v6 tubes at 4400 instead? I'm mostly just curious about what might happen.
I read that the typical el84 design doesn't like to have a large signal on its grid (compared to the 6v6) and a suggestion was made to me to change the tail resistor so that the phase inverter would pass more signal on to the power amp. Is there any possible drawback to doing this?
It seems like (if this works) this tail resistor would be a great place to put a pot. Wouldn't it be an excellent way to alter the sound of an amp that relies primarily on the power tubes for distortion?
Bonus question.
The OT is for 6l6 tubes. It's about 4400 ohms w/ 4, 8, and 16 ohm speaker taps.
I'm plugging an 8 ohm speaker into the 4 ohm winding. That ought to be 8800. How risky would it be to try running the 6v6 tubes at 4400 instead? I'm mostly just curious about what might happen.
I find the standard LTP PI in the Marshall 18W overdrives the EL84s too much (but that's just personal taste and others may disagree). For me, it might already provide enough drive for 6V6s.
Reducing the tail resistance will increase the output swing of the LTP. It will also increase the 'imbalance' of its two outputs (although that may not be a terrible thing in a guitar amp). You can try it with a pot, but it will have plenty of DC through it - so it will give a scratchy sound if you adjust it while the amp is on. Might be good to use a pre-set pot to find the sound you like and then measure it and replace it with a fixed resistor.
You could also play around with the bias resistor. Hotter bias will give more output, thus giving the 6V6s greater signal and overdriving them more. Colder bias will of course put out less signal and the LTP will distort. Some have said LTP overdrive closely resembles power tube overdrive, so this will get you distortion without the power tubes being driven so hard if you want. You could sub in a linear taper pot and wire it for variable resistance to try different settings. You'll have to figure out what value to put in for what you want to do with it.
As far as running the 6V6s into the 4400 ohm tap, it probably won't hurt the tranny since it's made for 6L6s but I would think you would stress the 6V6s because they see only about half the load they should see.
As far as running the 6V6s into the 4400 ohm tap, it probably won't hurt the tranny since it's made for 6L6s but I would think you would stress the 6V6s because they see only about half the load they should see.
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Much depends upon what limits at clipping. Assuming its in class B near clipping, things are normally devised so that the voltage dropped across the anode load causes the anode voltage to be so low that the valves begin not to conduct. If the anode load is halved, this happens at twice the conduction current - yippee more output power if the PSU can keep up and the 6V6 doesn't get too hot. However, twice the conduction current requires more grid drive, and if the grid drive required is too positive then the grid conducts and you get into grid clipping ie you can't achieve the extra conduction - or the grid drive circuit hasn't got enough headroom and clips.
The upshot is a crapshoot - normally things get stressed and/or hot and limit such that you don't end up with anything like twice the output power !
This has been more fruitful even than I thought it would be. I started to make some mental connections to other things I've read.
It's like that scene in the movie A Beautiful Mind when the ideas start to form in Nash's brain, except that I'm not coming up with anything brilliant.
Changing from NOS GE 6l6gc tubes to new JJ 6v6 tubes, the JJs break up a little earlier, seem to be easier to get feedback, and they chime a little more. But there's also hum that wasn't there before.
It's like that scene in the movie A Beautiful Mind when the ideas start to form in Nash's brain, except that I'm not coming up with anything brilliant.
Changing from NOS GE 6l6gc tubes to new JJ 6v6 tubes, the JJs break up a little earlier, seem to be easier to get feedback, and they chime a little more. But there's also hum that wasn't there before.
I think the increased hum probably means that the 6v6 tubes are not such a well 'matched pair' as the 6l6 tubes were.
I can attest that the 6L6 tubes measured 42 and 39 mA by the shunt current method. I didn't measure the JJs yet. It's a cathode biased amp with a B+ of 305 and a 250 ohm cathode resistor. I assumed I wouldn't hurt anything if I used it without re-biasing.
Is it possible that the OT could have more hum on the 4 ohm tap compared to the 8 ohm tap? My gut says no but my gut has been known to be wrong a lot.
Is it possible that the OT could have more hum on the 4 ohm tap compared to the 8 ohm tap? My gut says no but my gut has been known to be wrong a lot.
Well, the 6L6 balance looks OK. If you're changing the tube type, I would think it's worth checking the bias.
Regarding the 4 ohm v. 8 ohm hum question, there seems to be a few variables changing at the same time here, so I'm not sure. But I know a well-matched output pair will help minimise the hum (due to better cancellation in the push and pull sides).
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