Go Back   Home > Forums > >

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

01A question
01A question
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
Old 21st January 2021, 11:46 PM   #501
mcandmar is offline mcandmar  Ireland
diyAudio Member
 
mcandmar's Avatar
 
Join Date: Feb 2012
Location: Dublin
I think i answered my own question, i have been testing the amplifier with RMAA and seeing identical gain and measurements on both channels with the gyrators set to 3ma, and the bias voltages the same. Plates were reading 123v & 116v.

Ale if your reading this i need help, i think i have damaged one of my Gyrator boards.

I had both set to 3ma (measured across the 10ohm resistor R5), i went back to tweak it and one channel read 8ma. Didn't understand why so i quickly wound the adjustment down to 3ma again, and now i am seeing 75v on the plate which makes no sense. Double checked the bias & filament voltages are correct, and tested the tube which is fine.

Any idea what is going on? I suspect i may have damaged the BF862 in J2?
  Reply With Quote
Old 21st January 2021, 11:57 PM   #502
nashbap is offline nashbap  United States
diyAudio Member
 
Join Date: Jul 2012
Location: New Jersey USA
01A question
Quote:
Originally Posted by euro21 View Post
Anode voltage and bias voltage determines operating point (Ua, Ia at -Vg) ... at specified tube and at given condition.

If the tube changing (another tube, or aging), the operating point also changing.

There are no two identical operating point of two tubes, but if it not too far -relative to each other-, it's irrelevant.

The important thing is tubes gain -preferably- similarity at near operating points.

Do you prioritize matching gain over similarity in THD spectra or try to get similar THD spectra and then adjust the gain elsewhere, for example, at the voltage divider just before the SF?
Thanks . nash
__________________
F5TV3, AVC, BA3b pre, Salas DCG3b, OBL-15, SonyVfet2pair monos, Millett Nutube Bal pre
  Reply With Quote
Old 22nd January 2021, 12:28 PM   #503
Rod Coleman is offline Rod Coleman  United Kingdom
садовник
diyAudio Member
 
Rod Coleman's Avatar
 
Join Date: Jul 2004
Location: Warwickshire UK
01A question
Quote:
Originally Posted by mcandmar View Post
Theory question: I am using Ales Gyrator boards and Coleman filament regulators with cathode bias. What is more important to match, bias voltage or plate voltage?
With preamps, where the output swing is only a few volts, the DC Anode voltage needs only to be in the right range to yield a suitable bias level. So this is least important for tight control. The triode gm varies with anode current, and this is more relevant for good performance. So I would say aim for a matched anode current - and you can adjust the anode voltage and/or the bias voltage to get there, so long as they don't end up being wildly different.
  Reply With Quote
Old 22nd January 2021, 06:32 PM   #504
euro21 is offline euro21  Hungary
diyAudio Member
 
Join Date: Sep 2004
Location: Budapest
01A question
Quote:
Originally Posted by nashbap View Post
Do you prioritize matching gain over similarity in THD spectra or try to get similar THD spectra and then adjust the gain elsewhere
If you measured enough large batch of -nearly- thousand years old tubes, you will realise, that selecting two "identical" tube is a real nightmare, it's almost "Mission Impossible". :-)

The traders/vendors measuring "pairs" with tube testers.
The average -good- tube tester measures tube at given operating point where gm near -or not so near- to datasheet's value in this point.

I shows, that in this point (changing from one tester to another, no "fix" point of this tube) the tube condition near to required value or not ("good"-"not good", "%" etc.).

It says nothing from distortions, which is a function of given curves (Ua-Ia at given -Vg) to relative datasheet curves.

If you a lucky, and own a curve tracer (uTracer, eTracer etc.) which shows curves, you can compare curves from tube to tube, relative to datasheet's curves.

It' a "real" pairing, but slow, labour-intensive, costly (do you have few bunch of hundred years old tubes?).

Instead of this, the gm measuring at given operating point is accepted.

After this you can do THD spectrum analysis, but I say in advance that you will not two "identical" spectra, even measuring tight "pairs".

BTW some years ago I measured a few bunch of 801/801a tubes.
The difference of THD spectra was larger (better-worse), than measuring -good, but not excellent- 801a/VT-62 tube versus 25/VT-25!
  Reply With Quote
Old 23rd January 2021, 02:26 AM   #505
nashbap is offline nashbap  United States
diyAudio Member
 
Join Date: Jul 2012
Location: New Jersey USA
01A question
Thanks very much for this detailed response. Coming from Mosfets it shows how little I know about tubes. Will be fun!
nash
__________________
F5TV3, AVC, BA3b pre, Salas DCG3b, OBL-15, SonyVfet2pair monos, Millett Nutube Bal pre
  Reply to this post

Reply


01A questionHide this!Advertise here!
Thread Tools

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
6V6 / 6AH4 / 01A (UX201a) juanitox Swap Meet 0 26th May 2013 06:42 PM
01a DHT Preamp mogliaa Tubes / Valves 0 13th May 2012 11:07 AM
01A tube amp Schematic??? jmillerdoc Tubes / Valves 4 29th December 2009 02:39 PM
Can I use a 01A in a headphone amp? sbelyo Headphone Systems 27 28th April 2006 12:33 PM
questions about 01A / 483 pre-amp. RichJones Tubes / Valves 0 11th September 2003 09:46 PM


New To Site? Need Help?

All times are GMT. The time now is 05:29 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
Copyright ©1999-2021 diyAudio
Wiki