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Need help/advice troubleshooting a new DIY tube amp build
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Author:  three2em [ 02 Oct 2017, 19:17 ]
Post subject:  Need help/advice troubleshooting a new DIY tube amp build

Hey all. New here. I've been designing and building a guitar amplifier for the past couple of months, and finally got everything wired up this past weekend. It's based on the original Ampeg Reverberocket 12-r-12 using 6SL7/6SN7 tubes for the preamp, and a pair of 6V6s for the output tubes. I've attached a copy of my schematic just in case. The amplifier currently "works," but with some obvious issues. The first being that I have the volume control cranked all the way up just to hear anything (sound level similar to about a 2 on most amps.) Secondly, if I play a chord, or strum hard, the sound breaks up. No in a cool distorted way, but like it cuts out and farts, like a distortion pedal that's running low on battery.

I turned the amp on tonight to do some voltage/current readings, and this is what I'm getting:

-First off, the tube filaments are reading 7.3VAC, which obviously seems a bit high, and the filaments for the 5Y3 rectifier are reading 5.5VAC, again a bit on the high side.
The voltages read from the 6 filter capacitors are as follow:
1st - 277V, though never stabilized, and continued to decrease through out the 10 minutes or so I was doing the readings. 277V was the reading shortly after reading the voltages on the plates of the 6V6s.
2nd - 256V
3rd - 195.4V
4th - 188V
5th - 160.5V
6th - 124V

I then read the voltages and currents running through each of the plates of the pre-amplifier tubes/phase splitter tube.

6SL7 - Preamp/Tremolo
V1B - 122V, 10.34mA
V1A - 243.5V/96.5, 8.4mA (this is used for the tremolo, the first reading was with the tremolo off, second with it on.)
6SN7
V2A - 55V, 4.69mA
V2B - 59.3V, 5.07mA
6SN7 - Reverb
V3A - 163V, 13.57mA
V3B - 75.8V, 6.46mA
6SL7 - Phase Splitter
V4A - 147.5V, 12.3mA
V4B - 148.2V, 12.43mA
6V6 - Output tube
V5 - 245V, 10.125mA
6V6 - Output tube
V6 - 249V, 12.32mA
(The currents on the 6V6s were calculated by dividing the voltage drop across the plate/center tap of out transformer by the resistance between output primaries and output center tap.)

Looking over the datasheets for all three of the tubes used, it looks like my numbers are way off. But before going further to try and rectify the situation, I wanted to see if anyone here maybe had some advice or knowledge that could help me going forward with troubleshooting. This is my first amp build, so I'm a little lost as to how I can fix my problems. Thanks.

Author:  Alpha Wolf [ 04 Oct 2017, 08:22 ]
Post subject:  Re: Need help/advice troubleshooting a new DIY tube amp buil

Resistor lr5 is a to low value and is starving the 6sn7 tube. Also your feed back is out of phase. This is the main prolbem. I'm guitar amps of feed back it out of phase it will osiclate.causing a low rumble or even a high pitched squell. I have built a lot of guitar amps as well as HiFi amps over the past 46 years. Try sealing the plate leads on the 6v6 output tubes. That should take care of the feed back prolbem.

Author:  gofar99 [ 04 Oct 2017, 17:17 ]
Post subject:  Re: Need help/advice troubleshooting a new DIY tube amp buil

Hi, Looking at the readings....the current through the 6SL7 tubes is essentially off scale for their normal use. It is more in the range of 6SN7s. 6SL7s are usually run with a plate current below 5 ma. 6SN7s can go to 25-30. I know that guitar amps tend to use tubes in non-hifi ways, but those values if correct seem excessive. I use 6 and 12SL7s all the time and usually the current is in the 0.5 to 0.7 ma range. Comfortably within the most linear region. The tubes are also rated for a max of 1 watt dissipation. If your values are correct you are well past that. Looking at the schematic I believe the measurements are off.

Good listening
Bruce

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