Marshalls: Why did 600V B+ become 470V? - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

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

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 May 2008, 07:36 PM   #1
работник
diyAudio Member
 
Rod Coleman's Avatar
 
Join Date: Jul 2004
Location: Warwickshire UK
Default Marshalls: Why did 600V B+ become 470V?

Been restoring guitar amps from 1960s to 1980s, and noticed how the oldest Marshalls, Laneys, Oranges, Hiwatts all ran B+ at near to 600V for EL34-equipped models.

By the mid-1970s, many of these had dropped the B+ to 460..480V. Many opinions favour the older designs for sound, so why did they drop the supply tension? Was it just to cheap out on the trafo insulation materials?
  Reply With Quote
Old 15th May 2008, 08:05 PM   #2
coresta is offline coresta  France
diyAudio Member
 
coresta's Avatar
 
Join Date: Mar 2005
Location: Paris
No , just a reliability concern
  Reply With Quote
Old 15th May 2008, 08:12 PM   #3
GordonW is offline GordonW  United States
diyAudio Member
 
GordonW's Avatar
 
Join Date: Mar 2005
Location: Marietta/Moultrie GA
In essence, it was about the tubes available.

Early EL34s were rated at up to EIGHT HUNDRED volts B+.

However, later-production tubes (Russian/Chinese, primarily- but even some of the remaining US manufacturers in the late '70s and beyond, as tubes became a lower priority compared to solid-state) had a tendency to go up in sparks, at 600v or higher.

So, many amps were dialed back, to 500v or less.

It's about not having warranty claims, from tube issues...

Regards,
Gordon.
__________________
Speaker Design, Restoration and Repair- since 1985.
  Reply With Quote
Old 15th May 2008, 08:21 PM   #4
coresta is offline coresta  France
diyAudio Member
 
coresta's Avatar
 
Join Date: Mar 2005
Location: Paris
and don't mix "real " EL34's (Xf2 Mullards, Philips ...) with 6CA7
  Reply With Quote
Old 15th May 2008, 11:50 PM   #5
mikecj is offline mikecj  United States
diyAudio Member
 
Join Date: Dec 2007
Default EL34 B+

Timely thread for me. So guys asking for opinions here.

I am currently restoring a Bedrock 1200 which runs a pair of EL34 and has the plate current at 590 UNLOADED [no tubes]

Anyone have opinions on the current JJEL34 or KT77 re reliability at these voltages [ I know it will come down a bit when "loaded]

Have been trying to decide whether to replace the overheated screen supply 1k 15watt with a 25watt wire wound, move the 470ohm 5watt screen resistors up a bit to say 680ohm and leave the B+ where it is or to rework the PS to Lower the B+.

I would prefer the former if the current tubes will take it.

THOUGHTS AND ADVICE APPRECIATED

BTW Bedrocks were made in the 90s up in New Hampshire not much info on them out there and I suspect that reliability may have been their demise although they seem to be well built and thought out with the possible exce[ption of "pushing the envelope" literally. This one fell into my lap and I'd really like to get it up and running.

Mike
  Reply With Quote
Old 16th May 2008, 12:20 AM   #6
JandG is offline JandG  United States
diyAudio Member
 
Join Date: Jan 2005
Location: Washington State
Not much makes me more happy than the sound of a old Marshall about to blow...
  Reply With Quote
Old 16th May 2008, 12:21 AM   #7
diyAudio Member
 
Join Date: Jan 2008
JJ KT77 will take 700V easily, so I would vote for them. And go for 1K 5W screen resistors for more safety.
  Reply With Quote
Old 16th May 2008, 12:38 AM   #8
diyAudio Member
 
Join Date: Apr 2003
Location: Portland,Oregon
Blog Entries: 4
Send a message via AIM to DigitalJunkie
Surely replace that overheated screen resistor,bumping it up to a higher wattage would be a good idea.

The cathode bias,I dunno..hard to say without seeing a schematic.
  Reply With Quote
Old 16th May 2008, 03:19 AM   #9
cerrem is offline cerrem  United States
diyAudio Member
 
Join Date: Jun 2005
Location: San Diego, CA
Plate voltage has never really been the problem with blowing the tubes... It's the SCREEN voltage being too high....
Mike...If you have a voltage doubler circuit...take the choke lead that feeds the screen supply and stick it in the middle of the two voltage doubler caps.... This will knock down the screen voltage to the 300V range..... tubes will be mucho happier...and don't worry about the high plate voltage it won't do any harm....
You could try 1K 5W resistor on each screen to keep the screen grid current running cool....


Chris
  Reply With Quote
Old 16th May 2008, 07:53 AM   #10
работник
diyAudio Member
 
Rod Coleman's Avatar
 
Join Date: Jul 2004
Location: Warwickshire UK
Default Controlling the Screen voltage

I'd heard that ScreenGrids were weak on the recent Pentode & Beam Tetrode production.

The 1960s guitar amps put only a choke & a resistor between the 600V B+ and the screengrid, so this terminal gets about 570V. Even the mighty Mullard EL34 is rated at only Vg2=500V max, so sparks must certainly be expected from the weaker designed valves.

I offer my customers a reduced screen voltage, implemented with a voltage divider and (900V) power-FET source-follower. 400V Vg2 seems a lot kinder to the EL34s, and they sound really good like this too. Only downside is that the gain is reduced because lower Vg2 means lower gm. But you can usually fix this by adding a capacitor to any unbypassed preamp cathodes - there's usually at least one like this.

If you add 10uF/450V at the FET gate, you can do without the power supply choke, too - a FET gyrator gives excellent hum rejection at the screengrids.

Marshalls usually use 1K 5W at G2, combined with 5,6K at G1. Makes sense to follow that - even with 400V on the screengrids, I believe.

But if modern EL34s can be run OK at 600V/(400V G2), what about modern transformers? Modern Marshalls seem to have a less than perfekt record of trafo reliability. Anyone got experience running replacement manufacturers' trafos at 600V+?
  Reply With Quote

Reply


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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
FS: (4) .01uf/600V Auricaps $20 mrdon Swap Meet 0 27th October 2008 10:14 AM
FS: (4) .01uf/600V Auricaps mrdon Swap Meet 11 13th October 2008 04:17 AM
EH EL34's at +600V? DigitalJunkie Tubes / Valves 34 10th September 2008 01:34 AM
600V 2A rectifiers for sale I_Forgot Swap Meet 0 14th June 2006 06:36 PM
845 or 813 valves at 600V? James D. Tubes / Valves 16 20th March 2003 02:58 PM


New To Site? Need Help?

All times are GMT. The time now is 06:11 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2