DIY Audio Projects Forum
 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

DIY Audio Projects Forum

Welcome to the DIY Audio Projects Message Forum. Use these forums to discuss Hi-Fi audio and to share your DIY Audio Projects. Registration is free and required to post messages and view the file attachments. Registration will only take a minute and registered users do not see any advertisements. After you have completed the online registration process, check your email (including spam/junk folder) for the verification email to activate your account. New members are under moderation - so your posts will not be visible until approved by a moderator. See the Read Me 1st, Forum RULES and Forum FAQ to get started on the forum.

It is currently 07 May 2021, 04:17

All times are UTC - 6 hours [ DST ]

Post new topic Reply to topic  [ 3 posts ] 
Author Message
PostPosted: 02 Oct 2017, 19:17 

Joined: 01 Oct 2017, 13:11
Posts: 4
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.)
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.

This post has a file attachment. Please login or register to access it. Only Registered Members may view attached files.

PostPosted: 04 Oct 2017, 08:22 

Joined: 30 Aug 2017, 09:49
Posts: 46
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.

Alpha Wolf

PostPosted: 04 Oct 2017, 17:17 
Project Author
User avatar

Joined: 04 Jun 2008, 20:59
Posts: 4361
Location: Arizona, USA
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

Some of my DIY Tube Amplifier Projects:

Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 3 posts ] 

All times are UTC - 6 hours [ DST ]

Who is online

Users browsing this forum: Google [Bot] and 73 guests

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
DIY Tube Projects :: DIY Tube Amp Kits :: DIY Speaker Projects :: DIY Solid State Projects :: DIY IC / Op-amp Projects :: DIY Phono Projects :: DIY Cable Projects :: Hi-Fi Audio Schematics
© - Terms of Service - Privacy Policy