Design using EL42 output pentodes - Page 3 - 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 2nd September 2008, 01:02 AM   #21
mach1 is offline mach1  Australia
diyAudio Member
 
mach1's Avatar
 
Join Date: Sep 2004
Location: Brisvegas
Phil,

You could try looking for a pair of second hand 8k transformers and wire them up using the 4 ohm tap. This arrangement is less than ideal but may well sound ok.

if you are not happy with the results you could ditch the EL42s and use the transformers with tubes suited to an 8k primary (eg EL84)
__________________
Au rutti wop-bop-a-loom-bop-a-boom-bam-boom - Richard Penniman
  Reply With Quote
Old 2nd September 2008, 04:48 PM   #22
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
One comment: I'm not sure you could get a 12AV7 to work properly in a direct-couple voltage amp/split-load because of the low available B+. If we run the plate voltage of the first stage as low as 100V (and that's getting down near where the tube will be pretty unhappy), the second stage will have about 102V on the cathode. Likewise, the drop across the plate resistor will be about 102V, so with a max of 250V available, you're stuck with under 50V across the tube. it will NOT be happy.

You've got four choices, as I see it:

1. Use a different topology. A long tail voltage amp/phase splitter could work on the available B+ with a 12AV7 or a 12AT7.

2. Couple with a step network, i.e., a voltage divider with the upper resistor bypassed with a capacitor. If you choose a divider ratio to get you 70V on the grid of the split load inverter, that leaves 110V across the tube in that section, a much more reasonable operating voltage.

3. Capacitor couple from the voltage amp to the split load. That gives the most flexibility, but you'll have to take care about low frequency stability since you've introduced a third LF rolloff.

4. Use a tube that can tolerate lower voltages. The ECC88 comes to mind. If you have a copy of Morgan Jones's "Valve Amplifiers," it's worth working through the Bevois Valley input stage calculations. If you don't have it, time to buy it.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 2nd September 2008, 05:57 PM   #23
phil_2 is offline phil_2  United Kingdom
diyAudio Member
 
Join Date: Jul 2008
Thanks for the responses

Eli, hmmm that hammond transformer, the 'E' model, at least its not the smallest available... there are smaller and cheaper and although the frequency response is stated as 150 Hz - 15 Khz, this value is stated for the full 125 range, my thinking is that the larger, 15W model has to be better than the smallest 3W model, right?

I think regarding the OT situation that I will build the amplifier as planned leaving purchasing OT's until last, I've sent a few emails and I'm working through the transformer suppliers on the net, hopefully something of reasonable quality will come up second hand or just at a good price. In the mean time I have a working transformer that I can use to test the outputs one at a time which will be useful for de-bugging.

Mach1, regarding your comment about switching to EL84's in the future if I'm not happy with the OT's that I can get for the EL42's; could the rest of the circuit be left the same if this were the case? would my power transformers still be upto the job?

SY, thanks for the advice, option 2 seems the most appealing to me.... mainly because I understand it do you mind me asking what you would do if you were me?

and yes I've seen many recommendations on this forum for the Morgan Jones book, it is time I got myself one!
  Reply With Quote
Old 2nd September 2008, 07:43 PM   #24
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
If I were me (oh wait, I am!), I'd drill an extra hole in the chassis and use different tubes for the input and phase splitter. Or I'd build a second supply for the driver stage with a higher voltage and use a 12AT7.

But if I were you, I'd get "Valve Amplifiers" and a couple of ECC88s.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 3rd September 2008, 12:20 AM   #25
mach1 is offline mach1  Australia
diyAudio Member
 
mach1's Avatar
 
Join Date: Sep 2004
Location: Brisvegas
Quote:
Mach1, regarding your comment about switching to EL84's in the future if I'm not happy with the OT's that I can get for the EL42's; could the rest of the circuit be left the same if this were the case? would my power transformers still be upto the job?
Unfortunately not - the EL84s will require more current and a bit more drive from the preceeding circuitry (from memory the EL42 biases very low).

I second the idea of purchasing 'Valve Amplifiers'. The step through Beauvois Valley amp design process will give you quite a good idea of the design principles involved.
__________________
Au rutti wop-bop-a-loom-bop-a-boom-bam-boom - Richard Penniman
  Reply With Quote
Old 3rd September 2008, 12:46 AM   #26
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
SY is 100% correct about the 6DJ8/ECC88 working well at modest B+ rail voltages. 90 V. on the plate is "textbook". However the μ for the type is only 33. Could enough gain be obtained, without resorting to CCS loading?

I LIKE the 12AT7/ECC81 in PP amps. Pete Millett has provided us with the means to get a "tall" B+ supply for the 'T7, without adding any additional "iron". Some "sand" in the PSU gets the job done. Connect both plates of the EZ41 to the rectifier winding's CT. The B+ for the EL42s still gets taken from the EZ41's cathode. Connect a bridge rectifier made 4X UF4007s to the ends of the rectifier winding. Each UF4007 should be paralleled by a HIGH WVDC/10 nF. snubber capacitor. Ground the anode junction of the bridge. Take the "tall" B+ rail from the cathode junction of the bridge. The "short" B+ rail is full wave rectified, even though a quick look indicates otherwise. The path to ground for both rails is via the same 2X UF4007s. A Maida style regulator will set 12AT7/ECC81 B+ exactly to the value SY wants.


Quote:
Eli, hmmm that hammond transformer, the 'E' model, at least its not the smallest available... there are smaller and cheaper and although the frequency response is stated as 150 Hz - 15 Khz, this value is stated for the full 125 range, my thinking is that the larger, 15W model has to be better than the smallest 3W model, right?
Things will be difficult enough using the 125E. In order to extend that 150 Hz. down to something reasonable, a good sized NFB loop error correction signal will be needed. Such an error correction signal requires magnetic headroom. The cores in the smaller 125 series trafos are too small. AAMOF, to further reduce the chance of O/P core saturation, change the 510 KOhm grid leak resistors to 470 KOhms. The high pass "corner" freq. moves up, but stays below 30 Hz. The little bit of "bending" being done to the 1:10 impedance rule is probably of no consequence.

Speaking of NFB loop error correction signals, change the value of the cap. that connects the "hot" speaker wire to ground from 1200 pF. to 1500 pF. Miracles do not come forth from Hammond 125Es.

Converting to 100% SS B+ rectification might get the "short" B+ rail large enough to use EL821s as the O/P tubes and Edcor XPP15-8-8K O/P trafos. EL821s require 1 KOhm resistors in the triode tie positions.
__________________
Eli D.
  Reply With Quote
Old 3rd September 2008, 03:46 AM   #27
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
The light bulb turned on. Use EDCOR model XPP15-4-8K O/P trafos in combination with 8 Ohm speakers. Hopefully, the primary inductance will be sufficient.
__________________
Eli D.
  Reply With Quote
Old 3rd September 2008, 11:23 AM   #28
phil_2 is offline phil_2  United Kingdom
diyAudio Member
 
Join Date: Jul 2008
Thanks Eli, I was actually thinking the same thing with the OT, the correct ratio between primary and secondary windings is the important thing and that would still be kept, I would be very happy to use the Edcor transformers

now as for all these other driver stage options its giving me a bit of a headache! A few thoughts tho... as I am planning to build both channels into the same chassis, if an extra gain stage is needed (say using ECC88s) then at least it would only be one extra tube and not two (half for each channel). in which case if there isn't enough spare power for the extra tube then there are two options as I see it, add a THIRD power transformer, which would only have to be a tiddler, or replace the old transformers with one big one thats got enough juice for everything... reducing the only usable parts from my amps to the EL42's and their sockets! but I'm ok with this.

If I do replace the power transformer this will also open up my options for driver tubes as I could use a higher suply than 250V.

personally I don't mind using diodes instead of tubes for rectifiers but I would prefer to keep the level of 'sand' to that, I'm sure the above circuit you mention could work but I would prefer to keep things simple
  Reply With Quote
Old 3rd September 2008, 04:27 PM   #29
Merlinb is offline Merlinb  United Kingdom
diyAudio Member
 
Merlinb's Avatar
 
Join Date: May 2006
Location: Lancashire
Default Re: Design using EL42 output pentodes

Quote:
Originally posted by phil_2
Hi, this is my first post here but I've been reading this forum for some time and I'm learning by the day.
Hi Phil! I was just wondering what part of Wales you're from? I work in Powys, and thought I'd mention it if you ever need any practical help.
  Reply With Quote
Old 3rd September 2008, 09:24 PM   #30
phil_2 is offline phil_2  United Kingdom
diyAudio Member
 
Join Date: Jul 2008
Default Re: Re: Design using EL42 output pentodes

Hiya Merlinb, nice to find a local on here I'm living in Caerphilly, thats about 10 miles outside of Cardiff. Thanks for the offer of practical help, very kind of you. I think to be honest once these design hurdles are over the practical part will hopefully be fairly straightforward
  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
Output (beam) pentodes / tetrodes suitable for screen grid drive ilimzn Tubes / Valves 19 15th June 2013 06:23 AM
Pentodes. bigwill Tubes / Valves 23 4th August 2009 02:14 AM
John Broskie's headphone design using pentodes Brit01 Headphone Systems 14 15th July 2009 02:28 AM
Beam pentodes and suppressor-grid pentodes of same type jon_010101 Tubes / Valves 3 2nd May 2009 11:18 PM
Anyone try sharp cutoff pentodes in a p-p output stage? aletheian Tubes / Valves 3 28th November 2005 06:28 AM


New To Site? Need Help?

All times are GMT. The time now is 07:37 PM.


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