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

DIY CAT SL1 preamp
DIY CAT SL1 preamp
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 17th February 2013, 08:37 PM   #91
DF96 is offline DF96  England
diyAudio Member
Join Date: May 2007
3k is fine if that is the best that can be achieved: high anode resistor from a lowish supply rail. Given an active load we can do better by using a bit more current, so smaller Rk. Ideally approaching 1mA as that is where the mu curve becomes flat. The fact that the wrong value for the upper grid resistor gives less distortion (in a simulation?) suggests that the valves are running non-linear, which means the wrong bias point is being used.
  Reply With Quote
Old 18th February 2013, 07:36 PM   #92
HJWeedon is offline HJWeedon  United States
diyAudio Member
Join Date: Sep 2005
Hello from Salem MA.

There are at least one thing wrong with using a vacuum tube as a load in an amplifier.
My number one objection is that unlike transistors, indirectly heated vacuum tubes normally have a heater and a cathode. I know that the manufacturers spec is that there can be a significant voltage differential between the heater-coil and the cathode tube.

The way the cathode is constructed is: A small cylinder of nickel has a folded thin wire of usually tungsten inside. This tungsten wire has an insulating coating so that there effectively is an insulating barrier between the heater and the cathode.

This heater / cathode barrier represented a significant technological challenge in production. I learned from my Dad, who was an expert at these things that for a reliable design you do not apply more than a few volts between the cathode and the heater.

There is no realistic way of preventing a noisy current to flow between the actual cathode and the heater wire. If this voltage is much more than about 20V there will be a current flowing between the heater and the cathode terminal.

I know that in indirectly heated rectifiers the heater to cathode voltage is permitted to be as large as 450V DC. that is OK for a rectifier tube, because the power-supply is normally bypassed well.

In an application where we can expect relatively low signal voltages the unintended current between cathode and filament may become a major source of noise and signal.

I therefore firmly believe that using a tube as a load in a high quality preamp is counter-indicated as my patent lawyer would say.

Specifically, if the heater DC voltage is negative with respect to the cathode, the heater wire will emit electrons that lands on the cathode, this is going to be a noisy current because the heater is not designed to be an emitter, it just is.

Conversely, if the heater is more positive than the cathode, the heater will become the anode and again a noisy current flows.

This heater / cathode current-flow is not something new, but may resurface again because of lost knowledge. In other words: keep the differential; voltage between the heater and cathode as low a possible, and worry about the noise later.

Resistor load: no problem, tube load will become noisy with time.

Hans J Weedon.
  Reply With Quote
Old 18th February 2013, 09:25 PM   #93
DF96 is offline DF96  England
diyAudio Member
Join Date: May 2007
All the people who have looked into this seem to agree that lowest noise and longest valve life are given by making the heater positive, not zero as you suggest. The optimum appears to be about +40V, but it is a broad curve so the exact value is not too important. It does not act as an anode with respect to the cathode because the active cathode surface does not face the heater.

Active loads and cathode followers have been used without problems for a long time. At the output of an active load the signal has received significant amplification so noise will not be a huge issue anyway.

One proviso: it is said (by some) that NOS valves may have better heater-cathode insulation than modern valves.
  Reply With Quote
Old 20th February 2013, 07:03 PM   #94
HVfanatic is offline HVfanatic  Germany
diyAudio Member
Join Date: Feb 2008
DIY CAT SL1 preamp
Well, with this in mind and max. amplitude: No problem... ;-)
  Reply With Quote
Old 20th February 2013, 09:53 PM   #95
HJWeedon is offline HJWeedon  United States
diyAudio Member
Join Date: Sep 2005
Hi there.

Thinking back many years, I may very well be wrong in claiming the there should be no DC voltage between the Cathode and the Filament in a vacuum tube amplifier. Sorry about that. I remember back to the 4+1 Audio amplifier published by Phillips and Mullard in the 1950s. It used an EF86 pentode input amplifier and an ECC83 as phase inverter, driving a pair of EL86s in push pull.

Since the phase inverter was a long tailed pair the cathodes were biased up some 35V and the recommended DC bias for the heater centertap was indeed 40V. Under those bias conditions the filament in the indirectly heated cathode was always biased positive with respect to the nickel tube used as a cathode. My guess is that since the heater was hotter than the nickel cathode, no noisy emission current could flow from the heater to the cathode, and the cooler nickel cathode did not emit enough current to worry about.

I stand corrected. Thank you very much. When I am wrong, I am wrong.

Hans J Weedon

Salem MA.
  Reply With Quote


DIY CAT SL1 preampHide 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
Convergent Audio Tech SL1 preamp mobyd Tubes / Valves 6 17th September 2013 08:08 PM
CAT SL1 SIGNATURE schematik phono vog Tubes / Valves 1 16th September 2013 01:49 PM
Greetings and request for info on Cat SL1 preamp somi electronic Parts 2 16th September 2013 01:46 PM
CAT preamp marknoir Swap Meet 1 24th January 2007 02:55 AM
A Noisey CAT SL1 Reference hifiscratch Tubes / Valves 2 23rd February 2004 05:00 PM

New To Site? Need Help?

All times are GMT. The time now is 07:32 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