Gentlemen,
I've built 5 amps with help from here, 3 TW, 2 Marshall and all have run great, sound great!
I decided to put together a small amp for practicing and trying pedals out with. Simple two gain stage, vol/tone control/ LTPPI and two 6V6. Cathode biased.
Both stages are 100K, 1.5K/10uf, .022 coupling. Stage one, controls, stage two, PI, Output. Simple. Fender P-TF41318 25watt,6.6K OT. A Hammond 270FX PT at 550V and 175ma.
So everything looks good, kind of a Princeton front end, Deluxe back end so to speak. Went together pretty easy and came right on.
But after a tiny bit of clean all I get is overdriven sounds...I've changed tubes in both the preamp and the power section. Still the same.
So I made some measurements:
AC 589 Rect =373
Plates 365
Screens 330
PI 250 with 163 on the anode
V1 a&b 232 with 139 on the anodes
Cathode resisitor is 250, voltage is 21.65. 21.65/250 = .086
So 365 - 22 =343 x .086 = 29.5 Watts.
Thus is this a OT problem? 25 watt Fender getting 30 watts?
I seek guidance...new build.
Moderators: pompeiisneaks, Colossal
I seek guidance...new build.
In theory, theory is the same as practice. In practice it's different.
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martin manning
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Re: I seek guidance...new build.
I would want another 50v on the PI and preamp tube anodes. Do you have a large dropping resistor between the screen and PI power supply nodes? A full schematic will help.
Re: I seek guidance...new build.
Are you getting ~15 watts clean output power?
I build and repair tube amps. http://amps.monkeymatic.com
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Stevem
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Re: I seek guidance...new build.
measure the V+ on your first filter node with the amp at idle and note that voltage , then apply a test tone until the output starts to get distorted and then apply a bit more input signal, if the Voltage starts to drop off fast then that's a good indication that your OT has a short.
Another way to test out the OT would be if you can apply 10 vac to the primary side in which case you would then read .on its secondary side .22 volts on its 4 ohm tap, .31 volts on its 8 ohm tap and .44 volts on its 16 ohm tap.
When output tubes try to drive a OT with a short you will also see the cathode voltage go up and if you look inside the tubes in a darkened room you will see the inside of the tubes glow brighter as the pull more current from the power supply.
Another way to test out the OT would be if you can apply 10 vac to the primary side in which case you would then read .on its secondary side .22 volts on its 4 ohm tap, .31 volts on its 8 ohm tap and .44 volts on its 16 ohm tap.
When output tubes try to drive a OT with a short you will also see the cathode voltage go up and if you look inside the tubes in a darkened room you will see the inside of the tubes glow brighter as the pull more current from the power supply.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: I seek guidance...new build.
340V at 43mA is running a 6V6 a little hot, no?
Tube junkie that aspires to become a tri-state bidirectional buss driver.
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Stevem
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Re: I seek guidance...new build.
Yes, 43 ma is way way too hot even considering the cathode bias voltage change drop!
Good catch on that , I missed it!
This hot of a idle will effect how soon the outputs overdrive greatly!
With 21 volts on the cathode your tubes are really seing only some 320 Volts on the plate which inturn means each output should idle at no more then 24 ma by my math and if you want to push it to idling at 70%, then go for 26 ma each!
Good catch on that , I missed it!
This hot of a idle will effect how soon the outputs overdrive greatly!
With 21 volts on the cathode your tubes are really seing only some 320 Volts on the plate which inturn means each output should idle at no more then 24 ma by my math and if you want to push it to idling at 70%, then go for 26 ma each!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: I seek guidance...new build.
Martin
Yup big resistor, I’ll do up a schematic shortly but a 22k to PI and a 10k to the V1.
Yup big resistor, I’ll do up a schematic shortly but a 22k to PI and a 10k to the V1.
In theory, theory is the same as practice. In practice it's different.
Re: I seek guidance...new build.
xtian
Sure doesn’t seem like it but I’d have to get my scope out and check this...a whole other situation! Haven’t used it in ages...
Sure doesn’t seem like it but I’d have to get my scope out and check this...a whole other situation! Haven’t used it in ages...
In theory, theory is the same as practice. In practice it's different.
Re: I seek guidance...new build.
Steven
So sounds like a bigger cathode resistor is needed...
So sounds like a bigger cathode resistor is needed...
In theory, theory is the same as practice. In practice it's different.
Re: I seek guidance...new build.
couple of pics
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In theory, theory is the same as practice. In practice it's different.
Re: I seek guidance...new build.
70% is the 'magic' number we shoot for with a fixed bias amp. It's very common for a cathode biased amp to run at 100% plate dissipation. The 6V6 is good for 14W MAX so I'd say with these tubes running at 14.75W that's just fine.
Re: I seek guidance...new build.
Vp of 360V minus Vk of 20V means the tube sees ~ 340V. An Ip of 25-30 is appropriate.
The 6V6 is a 12W tube.
The 6V6 is a 12W tube.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
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martin manning
- Posts: 13274
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Re: I seek guidance...new build.
Id say about 5% of the cathode current is going to the screen, so it's not quite as bad as it seems. For a 6V6GT 12W based on cathode current seems reasonable, if it's a JJ 6V6S, 14W is fine.
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wpaulvogel
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Re: I seek guidance...new build.
It might be a good idea to put a voltage divider in place of that red bridge wire. That’s going to calm down some voltage. Two gain stages into a long tailed pair is lots of gain. I know there’s a divider on the first stage but it still gets jumped up.