Go Back   Home > Forums > > >

Tubelab Discussion and support of Tubelab products, prototypes and experiments

Reply
 
Thread Tools
Old 26th August 2021, 08:47 PM   #31
OldHector is offline OldHector  Sweden
diyAudio Member
 
Join Date: Jun 2018
Location: Täby, Sweden
The power supply has been adapted, and the voltages are as predicted, so that is a step forward. The plate voltage on the PL84 is still 216V, and now the ECC81 B+ is 275V, the plate has increased by 30%, 175V and 95v on the two anodes. I think this has filled out the sound a bit.

The next step is connecting the feedback. I tried a 5.1K resistor in series with a 390pF for starters, similar to the standard SPP, but I ended up with a mild hum, scratchy volume control, and some sort of crackling, or oscillation. The scratchy volume control is a sign of some grid voltage, so that probably means I need to calculate the resistor value a bit more smartly.

I am fairly confident it is not the wiring of the OPT - I have experienced the howling that can result from the OPT being wired out of phase, and it is not that type of noise.

Getting closer!
Attached Images
File Type: jpg PL84 PSU mk2.jpg (767.7 KB, 72 views)
  Reply With Quote
Old 27th August 2021, 08:14 AM   #32
OldHector is offline OldHector  Sweden
diyAudio Member
 
Join Date: Jun 2018
Location: Täby, Sweden
Should have been 'I tried a 5.1K resistor in parallel with a 390pF for starters' ...
  Reply With Quote
Old 26th September 2021, 01:35 PM   #33
OldHector is offline OldHector  Sweden
diyAudio Member
 
Join Date: Jun 2018
Location: Täby, Sweden
I could not get the feedback to work at all with the normal FB resistor value (5.1k). I'm a long way from understanding this. The FB resistor injects a proportion of the AC voltage from output signal to the cathode of the driver, and I would have thought that the ratio was related to the operating point of the driver tube and the output to the loudspeaker, which in my build should be more or less the same as a standard SPP.

In the end I used a 100k potentiometer and selected a point where the sound seemed nicest. This turned out to be around 43k, so I ended up fitting a 47k resistor just so I can move on to another project, and enjoy using the amp in the mean time.

The sound now does feel more precise, crisper, and the range is bit fuller, but it feels a little bit like designing using chicken entrails; it would have been nice to have been able to derive a value by simulation or calculation. Not having my oscilloscope does hamper things a bit.

I have a 490pF mica capacitor to fit at some point, but I wanted to listen to a variety of stuff first before trying that.
  Reply to this post

Reply


Tubelab SPP with EL86/6CW5Hide 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
anyone ever use el86/6cw5? mctavish Tubes / Valves 43 23rd April 2019 02:03 PM
does anyone know of a el86/6cw5 quad amp ? rifraf50 Instruments and Amps 16 6th June 2016 09:54 PM
Subbing EL86 (6CW5) for EL84 andyjevans Tubes / Valves 7 15th April 2013 03:45 PM
EL86 / 6CW5 availability for Simple P-P? hartze11 Tubelab 1 27th May 2010 03:16 AM
Spice model for EL86/6CW5 genome Tubes / Valves 0 17th October 2005 02:49 PM


New To Site? Need Help?

All times are GMT. The time now is 10:39 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
Copyright ©1999-2021 diyAudio
Wiki