Where are the 307A / VT225 amps? - Page 5 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 8th January 2009, 07:29 PM   #41
wicked1 is offline wicked1  United States
diyAudio Member
 
wicked1's Avatar
 
Join Date: Jan 2008
Location: Midwest
I'm a little confused about how to find the optimum current to run these tubes. The voltages we're running them at is off the charts (350 - 400v.. chart only goes to ~340). Is the idea to keep the same ratio, as if the line were on the chart?

My power transformer is supposed to be 350-0-350, but I'm getting over 400v at the tubes. I currently have the bias set to about 58ma. I'm not really using my amp right now because of the high voltage, but I plan to fix it this weekend.
I'm going to try getting my voltage down to 350. At that point I guess I'll try 65ma, like Mr TubeLab.

All of this is way off the charts, and I'm only doing it based on the couple of posts specifying this sort of info here on the forums.
So, just to double check... does that sound about right, or should I shoot for some other values while I'm working on the amp.

This is in a 300b s.e. being converted to 307a

thanks a lot!
  Reply With Quote
Old 8th January 2009, 07:57 PM   #42
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
Maybe someone has a better take on this, but to my eye, triode connected these look like neither 2A3's nor 300B's so much as 6L6's. Triode connected at least, Rp is slightly lower than a 6L6 (1K5 v 1K7) as is plate dissipation (15W v 19W), though the indication seems to be that pushing these a little is fine.

Conservatively, I think my starting point would be 310V on the plate, -30V on the grid, and ~45mA. Even pushing it to higher dissipation, that -30V on the grid seems about right to me.
  Reply With Quote
Old 8th January 2009, 08:00 PM   #43
coresta is offline coresta  France
diyAudio Member
 
coresta's Avatar
 
Join Date: Mar 2005
Location: Paris
Mine are 330 / 335v on plate and G2 according to the WE datasheet , and idle current 48mA (i.e. 33v bias on G1 )
  Reply With Quote
Old 8th January 2009, 09:43 PM   #44
diyAudio Member
 
pmillett's Avatar
 
Join Date: May 2005
Location: Dallas (but I am not a Texan!)
dsavitsk, you're right, the curves are pretty close to a triode-connected 6L6...

I've been running B+ of 400V and a shared cathode bias resistor of 500 ohms in push-pull... somewhere around 40-45V on the cathodes, I think, so between 40 and 45mA and ~360V across the tubes. So far they seem happy there.

Pete
  Reply With Quote
Old 8th January 2009, 09:57 PM   #45
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
Pete,

Any suggestions on output transformers? I see you are using a 20W 8K primary. There are lots of 15W 8K's out there, and a few 15W or 25W 10K's. Any sense on optimal load?

I have a pair of 5K 40W (LL1663) that I think I might breadboard with, but this seems sub-optimal to me.
  Reply With Quote
Old 9th January 2009, 01:19 AM   #46
diyAudio Member
 
pmillett's Avatar
 
Join Date: May 2005
Location: Dallas (but I am not a Texan!)
Quote:
Originally posted by dsavitsk
Pete,

Any suggestions on output transformers? I see you are using a 20W 8K primary. There are lots of 15W 8K's out there, and a few 15W or 25W 10K's. Any sense on optimal load?

I have a pair of 5K 40W (LL1663) that I think I might breadboard with, but this seems sub-optimal to me.
I haven't tried anything higher than 8k in P-P (I don't have any 10k's)... but less than 8K P-P probably isn't as good. In SE I found 5k is better than 3.5k, so I would guess that 10k P-P would work well.

The most power I've been able to squeeze out of a P-P pair is around 12W @ 5% THD, so I think you'd be OK with a 15W transformer.

Pete
  Reply With Quote
Old 31st January 2010, 06:01 PM   #47
diyAudio Member
 
Join Date: Apr 2009
Small 307A amplifier with 5K output trannies from old radio receiver:

307a Single End Amplifier

Dragan
  Reply With Quote
Old 23rd April 2010, 01:14 AM   #48
Pyre is offline Pyre  Canada
diyAudio Member
 
Pyre's Avatar
 
Join Date: May 2006
Send a message via MSN to Pyre
Quote:
Originally Posted by radiostar View Post
Small 307A amplifier with 5K output trannies from old radio receiver:

307a Single End Amplifier

Dragan
Do you have a schematic? I got a couple RK-75's that I want to try out SE
  Reply With Quote
Old 7th November 2011, 06:50 PM   #49
jrenkin is offline jrenkin  United States
diyAudio Member
 
Join Date: Nov 2007
Location: Denver, Colorado
I resurrected this thread to see if George or any one else had more insight into the 307A. I am thinking of subbing into a 300B tublab SE. Seems like George just swapped them in, meaning B+ of 400V and bias 60mA or so I suppose. Seems hot for these tubes by the WE data sheets.
  Reply With Quote
Old 7th November 2011, 10:41 PM   #50
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Quote:
Seems like George just swapped them in, meaning B+ of 400V and bias 60mA or so I suppose.
They are pentodes with a 5 pin base. I made a crude adapter wired them into my TSE, set the bias to about 60 ma and let it rip. That was more than 2 years ago and it is still running. It doesn't see a lot of use because it was never built into a proper cabinet, but it sounds good. I am not running them at 400 volts though. My TSE has 300-0-300 power transformer. It is a Hammond so it is really a 325-0-325 and the B+ is about 350 to 360 volts depending on the whip of the local power company at any given moment.

The WE spec seems rather conservative at 15 watts plate dissipation and I ran them at 30 watts for a few days, but they sound about the same at 20 to 25 watts.

On a slightly different note another poster in this thread that might be named Pete made this big red board for use with sweep tubes. I got tired of playing with sweep tubes so I stuck some 307A's in the red board. It works, makes about 30 WPC without melting anything. Sounds good too. I haven't had time to play with it in a few months, but I will get back to it.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
For Sale: Tube Amps, Class-A Amps, BG Ribbons, Chassis and Other Goodies! opc Swap Meet 23 2nd November 2009 06:04 PM
Big sale thread: PHL, Raven, Amps, In-wall/ceiling speakers, Amps, VideoRequest mbcouple Swap Meet 2 27th October 2008 11:27 PM
SMPS for diy audio amps and multi-channel diy amps (and their power supplies) tlparker Solid State 18 31st August 2006 11:39 AM
Why do some Bogan amps sound so good as guitar amps? amperex Instruments and Amps 7 22nd June 2006 12:46 PM
Has anyone compared GC amps(LM3875/3886/4780) with other DIY SS amps(Leach,ESP) ? percy Chip Amps 31 13th January 2006 02:33 AM


New To Site? Need Help?

All times are GMT. The time now is 01:57 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2