Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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

What do you think of this circuit
What do you think of this circuit
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
Thread Tools Search this Thread
Old 6th December 2017, 03:39 PM   #11
petertub is offline petertub  Sweden
diyAudio Member
Join Date: Jul 2015
Location: gothenburg,sweden
Originally Posted by Doppler9000 View Post
Is the circuit dangerous?
The 300B lives outside it's designed area. It still might work. But you cannot
complain if the tube goes into thermal runaway.

There is no point in using the tube outside it's spec, one could easily change
the grid resistor to be within design limits.
My home is at www.tubular-well.se
  Reply With Quote
Old 7th December 2017, 11:21 PM   #12
Miles Prower is offline Miles Prower  United States
diyAudio Member
Miles Prower's Avatar
Join Date: May 2005
Location: USA
Originally Posted by Doppler9000 View Post

I came across this circuit and wondered what people thought before I dig in further on the design. It is a regulated 300B SE unit.

There are a few problems:

1) The series capacitors in the power supply need 100K equalizing resistors. The tolerances of electrolytics isn't tight, and the DC will divide unevenly. This could lead to overvoltage across one or the other in the series string.

2) There are two capacitor couplings, both with 0.1uF capacitors. That will cluster the low frequency phase shift, possibly leading to low frequency stability problems once the gNFB loop is closed. Capacitor couplings need to be staggered to prevent that.

3) The input isn't protected against DC offset. You need an input coupling capacitor unless you're certain you won't be connecting to anything that could have DC offset.

4) The spec sheet for the 300B lists 250K as the max value for a grid return resistor. Here, you have almost twice that value. This could lead to a run-away condition. That's why I'd like to see a separate grid driver. A follower eliminates that problem, as well as the heavy load on the final gain stage. Triode gain stages don't drive heavy loads gracefully without distortion.

Also, complaints about disappointing sonic performance of 300B's is almost always due to designs with inadequate grid drive. Even if you're nominally operating in Class *1, these power triodes need adequate grid drive both for Ci + CMiller, but also for grid current, as grid current will flow even though Vgk < 0V.

5) I would lose that head jack and its associated switch. A failure there could leave the secondary of the OPT without a load, and that's a disaster waiting to happen. A plate current cutoff will let the OPT free wheel like a TV HD xfmr. Unlike a TV HD coupling xfmr, the OPT wasn't designed to stand up to K-volts, and you could lose the OPT. Also, decide what your load is, and connect the gNFB loop to that point. Connecting to the top of a multi-tap secondary with only part of it connected across the load gives stability problems.

Originally Posted by petertub View Post
unnedded Standby switch could be eleminated.
Not for a solid state power supply. The voltage will come up very fast even before the cathodes have a chance to warm up. It's better to allow for the application of heater power, let the cathodes warm up, before turning on the HV. That's how I do designs where SS power supplies are used. You could avoid the problem if the HV supply were redesigned to use hollow state power diodes.
There are no foxes in atheistholes

Last edited by Miles Prower; 7th December 2017 at 11:28 PM.
  Reply With Quote


What do you think of this circuitHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
vintage amplifier circuit board with TDA2010 with this tone/volume control circuit Peters8 Chip Amps 16 4th March 2018 07:17 PM
Op-Amp Circuit Components: Any Ideas About This Circuit Section's Function? Travis Clarke Analog Line Level 5 14th September 2016 01:30 PM
Building an analog circuit to replace the SS circuit in Marshall MG15dfx HCCourt Instruments and Amps 1 14th February 2014 04:10 PM
With painted a circuit, STI(2SK182E) pipe of the SRPP circuit, is feasible? lgqliu Solid State 2 25th February 2012 06:07 PM
Trouble with speaker protection circuit (Randy Slone's circuit) whalefat Solid State 3 13th April 2005 10:13 AM

New To Site? Need Help?

All times are GMT. The time now is 01:36 AM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio