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

Vintage console amp - suggestions to convert to standalone?
Vintage console amp - suggestions to convert to standalone?
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 15th September 2020, 03:27 AM   #1
slave2music is offline slave2music
diyAudio Member
 
Join Date: Aug 2020
Default Vintage console amp - suggestions to convert to standalone?

Found another vintage tube console that seems like a good candidate for converting into a standalone amplifier. I really like these old vintage consoles for honing my soldering and chassis modding skills. They usually come very cheap (sometimes free) and the accessible point to point wiring makes them fairly noob friendly.

Here's what I've done so far:
- removed the pre-amplifier / tuner section
- added polarized power cord, on/off switch, and fuseholder
- replaced all power filter caps (kept old capacitor cans for aesthetics)

Please note that all the tone controls, etc. are removed along with the pre-amplifer section. The pots and switches were not worth cleaning and using.

What should I do with the rest of the circuit to use it as standalone amplifier and have it sound good? Is it as simple as replacing the coupling caps as per the schematic? Or are other values or modifications recommended?

Any guidance much appreciated. Schematic attached.
Attached Files
File Type: pdf electrohome el84 amp.pdf (537.7 KB, 86 views)
  Reply With Quote
Old 15th September 2020, 03:47 AM   #2
jjasniew is online now jjasniew  United States
diyAudio Member
 
jjasniew's Avatar
 
Join Date: Feb 2002
Location: Roy, WA
Well, I guess I'd just bring it up to original health and enjoy the couple-few watts of single ended tube amplification.

Check the carbon composition resistors to see if any have drifted waaay off - you know about replacing the power supply and coupling caps already. If you find a similar vintage push-pull 6BQ5 amp, you could bring that one back to stock - and keep the two amps for a sound comparison shootout with your friends. They will certainly sound different -
__________________
Joe Jasniewski - PC>ZOUDIO AIO4CH> OB Lii Audio F15s w/ 18" assists.
  Reply With Quote
Old 15th September 2020, 04:42 AM   #3
Eli Duttman is offline Eli Duttman  United States
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
I will not pull punches. The "Electrohome" unit is a PoS. The schematic shows circuitry that's unsafe, non-linear, and exhibits poor voice coil control (damping factor).

C19 is an instance of the notorious "death" capacitor. The O/P tubes are in full pentode mode and no NFB is present. Hence, distortion and a high O/P impedance are present. YUCK!

Because the power transformer had to energize things other than the O/P section, it appears to be competent. The 6CA4/EZ81 rectifier can handle up to a 150 mA. draw and would not be present, if a less capable type was remotely up to the task.

Salvage the power transformer, and tubes (if still good). A 12AX7 section as voltage amplifier and full pentode 6BQ5/EL84 "final" gets you approx. 5 WPC. You need a global NFB loop and decent O/P "iron". Edcor's GXSE15-5K meets the requirements and will give you good bass extension, courtesy of mass and the NFB.
__________________
Eli D.
  Reply With Quote
Old 15th September 2020, 08:49 AM   #4
6A3sUMMER is offline 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
Using grid leak bias on the input tube is not a great way for consistent performance, nor for low distortion performance.

Put a self bias resistor and bypass cap from the input tube cathode to ground.
Repeat for the other input tube cathode.

Most modern signal sources have very small DC offset voltage.
And most modern signal sources can drive 10k loads.
So get rid of all that input circuitry.
Use a 50k pot for volume control, and a 1k grid stopper at the input grid.
That will keep the input tube Miller Capacitance at bay (50k versus 1Meg input pot).
And get rid of that cap from the input tube plate to ground (cuts off high frequencies).

Just like Eli said, get a decent output transformer, and add some negative feedback.
When you take out C19, take out R23 too.

Just my opinion.

Last edited by 6A3sUMMER; 15th September 2020 at 09:01 AM.
  Reply With Quote
Old 15th September 2020, 01:02 PM   #5
slave2music is offline slave2music
diyAudio Member
 
Join Date: Aug 2020
Quote:
Originally Posted by Eli Duttman View Post
I will not pull punches. The "Electrohome" unit is a PoS. The schematic shows circuitry that's unsafe, non-linear, and exhibits poor voice coil control (damping factor).

C19 is an instance of the notorious "death" capacitor. The O/P tubes are in full pentode mode and no NFB is present. Hence, distortion and a high O/P impedance are present. YUCK!

Because the power transformer had to energize things other than the O/P section, it appears to be competent. The 6CA4/EZ81 rectifier can handle up to a 150 mA. draw and would not be present, if a less capable type was remotely up to the task.

Salvage the power transformer, and tubes (if still good). A 12AX7 section as voltage amplifier and full pentode 6BQ5/EL84 "final" gets you approx. 5 WPC. You need a global NFB loop and decent O/P "iron". Edcor's GXSE15-5K meets the requirements and will give you good bass extension, courtesy of mass and the NFB.
Well, thanks for your honest feedback. Rather than in investing in O/P transformers and redesigning I think I'll just scrap the project, probably should have reached out before doing the chassis work. Anyway, lesson learned.

In applying that lesson I'm posting a schematic for another console pull that I have waiting in the wings. Same manufacturer but this time a push pull el84 with what seems like some decent iron?

What are your thoughts on this one? Suggestions before I start into the chassis updates for IEC, power, safety updates, etc.?

Thanks.
Attached Files
File Type: pdf console el84 pp.pdf (581.1 KB, 43 views)
  Reply With Quote
Old 15th September 2020, 01:59 PM   #6
IlikeTech is offline IlikeTech  United States
diyAudio Member
 
Join Date: Apr 2020
Location: Maine
I don't know what other people think, but the design in the PDF you posted looks a lot better.
  Reply With Quote
Old 15th September 2020, 02:27 PM   #7
rdf is offline rdf  Canada
diyAudio Member
 
rdf's Avatar
 
Join Date: Jun 2004
Location: big smoke
Given your stated goals and the time investment so far it might be worth the extra few steps of converting the front end to cathode bias and wrapping feedback from the opt secondary around it. Just for play. Recommend not sweating 'parts'.
__________________
Ears aren't microphones.
  Reply With Quote
Old 15th September 2020, 03:20 PM   #8
audiowize is offline audiowize  United States
diyAudio Member
 
Join Date: Aug 2005
Location: Seattle
Vintage console amp - suggestions to convert to standalone? Send a message via AIM to audiowize
Is this for guitar or hifi use? Ditching the tone controls and adding some feedback *or* triode strapping the 6BQ5 would help performance for Hifi.
  Reply With Quote
Old 15th September 2020, 03:50 PM   #9
Windcrest77 is offline Windcrest77  United States
diyAudio Member
 
Join Date: Feb 2019
Location: Lockport, IL USA
Quote:
Originally Posted by IlikeTech View Post
I don't know what other people think, but the design in the PDF you posted looks a lot better.
Ditto that, very simple and straightforward textbook circuit. Needs rework on the mains side for safety, ground, fuse, etc. That power transformer is wound for a lot of 6.3 volt heaters! For this reason I'd maybe look at replacing the power transformer and re-purposing it to some future project that needed a lot of heaters. If the OPT's are hefty you have something there, (I think).
  Reply With Quote
Old 15th September 2020, 04:26 PM   #10
jjasniew is online now jjasniew  United States
diyAudio Member
 
jjasniew's Avatar
 
Join Date: Feb 2002
Location: Roy, WA
Quote:
Originally Posted by slave2music View Post
I think I'll just scrap the project Thanks.
You could take the circuit suggestions provided and still make it a working single ended tube amp -

Let's say you did all the input tube recommendations, got rid of the tone stuff between the tubes, wired the output tube triode and put some feedback - and got 1W from 100Hz on up to as high as you can hear, with a couple - few percent of 2nd harmonic distortion...

Put that with some of the horn efficiencies I've seen around here (where folks are using a fraction of a watt at normal listening levels...), use the push-pull 6BQ5 amp for the bottom with an active crossover driving both.

Who do you know that has an all tube bi-amplification system with single ended on top, push pull on the bottom?

You read here about people going to all lengths to get their push-pulls operating as single ended - so there must be something to it. Perhaps the stock output tranny would be just fine - depending on how you use it.

The beefy power transformer - intended to power other now non existent circuitry - doesnt hurt.
__________________
Joe Jasniewski - PC>ZOUDIO AIO4CH> OB Lii Audio F15s w/ 18" assists.
  Reply With Quote

Reply


Vintage console amp - suggestions to convert to standalone?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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Vintage Stereo Console needs new speakers geejayen Multi-Way 21 6th December 2018 10:17 PM
Vintage Console Subwoofer Conversion cogitech Subwoofers 6 11th April 2013 11:35 PM
DIY New Vintage Stereo console Hollyrock Everything Else 1 7th September 2012 07:14 AM
Vintage radio console parts ElectricMan Swap Meet 3 28th December 2010 06:47 PM
vintage console in Steath Mode peace brainerd Analogue Source 0 6th May 2006 04:24 PM


New To Site? Need Help?

All times are GMT. The time now is 06:41 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Copyright ©1999-2020 diyAudio
Wiki