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 3rd April 2010, 02:49 PM   #1
diyAudio Member
 
Join Date: Dec 2009
Default My first amp project, 6BQ5 (EL84) Single-End

Well, after having aimed too high with an 845 Single-End, followed by a 300B Push-Pull, I concluded that I'm too much of a novice to be building such designs, so for my first ever tube amp project, I have decided to start small and easy.

I found a schematic for a 6BQ5 Single-End stereo design which seems straightforward. I have a few questions that I thought maybe some experts here would be gracious enough to provide guidance on.

1. In the power supply circuit the 5V4GA rectifier tube was selected for its relatively long warmup time, allowing the other tube filaments to warm up before blasting their plates with the B+ high voltages. This is however a hard-to-procure tube, so what should I look out for in the circuit when trying to find a substitute?

2. For the amp itself, what kind of input is expected? Do I need a preamplifier of some kind, or can I just hook up a CD player to it?

3. What is the single-triode equivalent of a 12AX7, since I'd like to have a single triode for each channel instead of using one 12AX7 to drive both channels.

4. The 6BQ5 pentode can be wired as a triode, too. Why would I want to do this? Triode configuration offers less power, but will it sound better? Triode mode requires R10 without R12 and pentode mode is the opposite, requires R12 while R10 should be omitted, if I understood the schematic correctly. Can someone confirm this?

5. Does the output transformer have a polarity or orientation between the primary and secondary windings that I need to pay attention to?

Any other design suggestions for improvements would be welcome, I haven't turned on my soldering iron yet!

Many thanks.
Attached Images
File Type: jpg 6BQ5 Single-End Power Supply.jpg (262.4 KB, 1103 views)
File Type: jpg 6BQ5 Single-End.jpg (169.6 KB, 1155 views)
  Reply With Quote
Old 3rd April 2010, 05:09 PM   #2
diyAudio Member
 
Join Date: Apr 2002
Location: Cayman Islands
1. In the power supply circuit the 5V4GA rectifier tube was selected for its relatively long warmup time, allowing the other tube filaments to warm up before blasting their plates with the B+ high voltages. This is however a hard-to-procure tube, so what should I look out for in the circuit when trying to find a substitute?

Maybe I am a bit high from inhaling the paint I was using a little while ago but I would suggest another circuit as this one does not quite "gel" with me
but anyway I would keep it simple and use a pair of ss diodes instead of the tube rectifier. This thing uses a choke input design.

2. For the amp itself, what kind of input is expected? Do I need a preamplifier of some kind, or can I just hook up a CD player to it?

You can attach a CD player as is, the variable resistor (R4) at the front works as a passive volume control.

3. What is the single-triode equivalent of a 12AX7, since I'd like to have a single triode for each channel instead of using one 12AX7 to drive both channels.

A single 12ax7 would be all you need, there are two triodes in each tube, one tube works for both channels. I don't know off hand of a single equivalent, if you really wanted 2 tubes then you could use two 12ax7's and just use one triode from each tube.

4. The 6BQ5 pentode can be wired as a triode, too. Why would I want to do this? Triode configuration offers less power, but will it sound better? Triode mode requires R10 without R12 and pentode mode is the opposite, requires R12 while R10 should be omitted, if I understood the schematic correctly. Can someone confirm this?

This is amplifier appears to be both triode and pentode connected at the
same time
R10 would be the triode connection and R12 the pentode connection. It cannot be both at the same time so you would have to add a switch to choose one or the other. Some people prefer "triode mode" some don't, with a switch you could have it either way. If connected as a pentode then you should ideally have a regulated supply for the G2 or failing that a voltage divider or failing that at least a decoupling cap on R12.


5. Does the output transformer have a polarity or orientation between the primary and secondary windings that I need to pay attention to?

I do not understand the question but the transformer is colour coded and there are usually two wires for the primary (although this particular model has three wires the third allows an "ultralinear" connection), one blue which would attach to the tube anode and a brown which would attach to your HV supply.
The secondary connections are for the speaker connectors, usually black for negative and another colour for the positive connection depending on your speaker, yellow for 8 ohm speakers.

Any other design suggestions for improvements would be welcome, I haven't turned on my soldering iron yet!

A 10W cathode resistor for an EL84 is more than overkill as is the 1628SEA for the output transformer.

Many thanks.

Your welcome.

I will not suggest a particular circuit but try
Audio Electronics Page
for a selection.

I hope this helps.
__________________
None of us are leaving this world alive. Enjoy it responsibly as you may.

Last edited by Andrewbee; 3rd April 2010 at 05:12 PM.
  Reply With Quote
Old 3rd April 2010, 05:25 PM   #3
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
1. This is to prevent "Cathode Stripping" which does not appear to be an issue at these voltages. It is a problem above 500V (IIRC). So you have the option of going SS for the rectifiers.

2. I am running a similar gain, and don't need a pre-amp. In fact I had too much gain (LOUD) till I added 8dB of GNFB. I think 6dB is probably a better value and I need to experiment with it still.

3. Not sure. Someone else will hopefully chime in.

4. Lower distortion in triode mode, but less output power as well. Yes that is correct on R12 and R10. Different harmonic spectrum as well.

If you are going to add Global Negative Feedback (which reduced distortion but also gain) you need to pay attention.

This schematic shows none so the transformer polarity does not matter.

6. That transformer is WAY over-rated. You only need a 5W transformer. I haven't done much work with SE amps so I'll defer to someone else on recommendations for an OPT.

Andrew beat me to it, I'm a slow typer.

Last edited by TheGimp; 3rd April 2010 at 05:28 PM.
  Reply With Quote
Old 3rd April 2010, 05:29 PM   #4
Svein_B is offline Svein_B  Norway
diyAudio Member
 
Join Date: Oct 2005
1. There is no need to worry about the tubes getting B+ before they are warmed up, as long as coupling caps are rated for the high voltage. I also find your PS to be a bit overkill. These tubes do net need DC for the filaments, and smaller chokes will do OK.

2. You will have enough gain for CD input without a preamp, even if you should select to replace the 12AX7 with a 12AT7.

4. If you run the EL84 as pentode you will need some feedback. Otherwise it will have terribly high Z-out, and sound bad. Feedback can be global, or local anode-to-grid as in the RH84 amp (described in other threads).

Svein
  Reply With Quote
Old 3rd April 2010, 05:44 PM   #5
diyAudio Member
 
Join Date: Dec 2009
Thanks for the tips, Andrewbee. My goal is to build an amp with as much "iron" and "glass" as possible and to lay out everything symmetrically. Even though this goal will probably be realized when I work my way up to a big 300B design or an 845. I want to keep the tube rectifier. If the 5V4 is too scarce I'll try another design.

You are correct that the schematic as is shows both the triode and pentode wiring, one or the other will have to be removed in the final construction. I didn't think of installing a switch, good idea, this way I can hear the difference!

For the output transformer then, the primary must be wired blue to plate and brown to B+, ok, got it.

You are correct that I over-indulged with the output transformer model, since a Hammond 125CSE would do just fine, but I wanted the enclosed type since it is meant to be exposed and the 125CSE as far as I can tell is an open-bracket type. The power transformer and the chokes are also more beefy than the orignal design called for.

Hey thanks for that link to those circuits, I actually own most of the Led Magazine issues from issue #118 or so, and now Electronique Pratique which has taken over. Those guys put out a large number of designs, but I always had trouble understanding all the details (they're in French), and I won't commit to building anything until I am 100% sure that I understand it all first.
  Reply With Quote
Old 3rd April 2010, 05:48 PM   #6
diyAudio Member
 
Join Date: May 2007
Greetings from FixitLand!

>3. What is the single-triode equivalent of a 12AX7, since I'd like to have a single triode for each channel instead of using one 12AX7 to drive both channels.<

6AV6 is a rough equivalent (=100), in a 7-pin base, with two diodes to boot (just tie 'em to the cathode). I agree with other posters that you're better off using one 12AX7 bottle with one triode going to CH-1 and the other CH-2. One less tube socket, half the heater power (12AX7 heaters in parallel = 300 mA; two 6AV6s = 600 ma). Plus, the 6AV6 isn't as hum-resistant as a 12AX7-A (or a 7025). But 6AV6s are cheap. And you're running the heaters off DC, so hum isn't a big concern; dunno if that's really necessary in this instance, though -- and with no regulation, you're probably gonna get a bit more than 6.3 VDC from that heater supply.

Take care,
--
J. E. Knox 'The Victor Freak'
  Reply With Quote
Old 3rd April 2010, 06:06 PM   #7
diyAudio Member
 
Join Date: Dec 2009
Ok thanks everyone for your suggestions. I'm leaning towards triode mode, with a switched-in pentode configuration (although I'm not sure how to connect the feedback exactly).

I already own modern sound systems, I'm not building this because I need it, just for the fun and enjoyment of having tube-based equipment. I have already restored and rebuilt tube radios for the same reason. There is about $540 worth of iron alone in this design, and I can probably do with half that cost, since they're all overrated, but I want to get a nice beefy look. This will be a springboard to an ultimate large project, probably 300B or 845.

I ordered most of the parts, I'll post photos on my blog of my progress and finished product once (and if) I get there. :-) I'll keep checking in here if anyone adds anything. Thanks again.
  Reply With Quote
Old 3rd April 2010, 06:17 PM   #8
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
OK, I see why they chose an over-rated transformer. I suspect the reason they chose that transformer is the frequency response.

What quality sound are you shooting for?

There are several OPTs you could use.

Edcor XSE10-8-5K limits frequency response to 70Hz - 18KHz. GNFB may improve this slightly. Inexpensive at $17.22 plus shipping.

Edcor GXSE10-8-5K slightly better freq response at 40Hz - 18KHz. Again GNFB may improve this slightly. Slightly more at $26.10 plus shipping.

Edcor CXSE25-8-5K slightly better freq response at 20Hz - 20KHz. Again GNFB may improve this slightly. Slightly more at $83.50 plus shipping.

Hammond 1628SEA 20-20KHz, $129.10 (AES).

Hammond Universal SE 125ASEA (25mA 3W) $49.65 (AES) Probably too small 100Hz-15KHz.

Hammond Universal SE 125BSE (45mA 5W) $50.45 (AES) 100Hz-15KHz.
  Reply With Quote
Old 3rd April 2010, 07:39 PM   #9
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally Posted by Cifrocco View Post
1. In the power supply circuit the 5V4GA rectifier tube was selected for its relatively long warmup time, allowing the other tube filaments to warm up before blasting their plates with the B+ high voltages. This is however a hard-to-procure tube, so what should I look out for in the circuit when trying to find a substitute?

You can replace the 5V4 with a 5U4GB. It's a large signal, dual diode with characteristics very similar to the 5V4. The main difference is that the 5U4GB is directly heated. I used the 5U4GB in a project when I needed to drop about a hundred volts to get to the design nominal DC rail voltage with the power xfmr I had on hand. Replacing hollow state diodes with solid state is likely to lead to serious over voltage.

2. For the amp itself, what kind of input is expected? Do I need a preamplifier of some kind, or can I just hook up a CD player to it?

Looks like there is more than enough gain for a CD player

3. What is the single-triode equivalent of a 12AX7, since I'd like to have a single triode for each channel instead of using one 12AX7 to drive both channels.

The 6SF5 is a singleton triode that's a near equivalent of one section of a 12AX7. (if you can find a 6F5, it's the same tube internally, but with a funky grid cap connection.) This one has a metal envelope, and requires an Octal base. There aren't very many singleton triodes in either seven or nine pin mini format.

4. The 6BQ5 pentode can be wired as a triode, too. Why would I want to do this? Triode configuration offers less power, but will it sound better? Triode mode requires R10 without R12 and pentode mode is the opposite, requires R12 while R10 should be omitted, if I understood the schematic correctly. Can someone confirm this?

Triode connection will definitely give less output. However, it just might be better sonically in this design which does not include any NFB. Pentode operation without NFB can lead to lots of nastiness, sloppy bass due to woofer underdamping, and poor bass due to a lack of OPT primary inductive reactance at the low end of the audio band.

5. Does the output transformer have a polarity or orientation between the primary and secondary windings that I need to pay attention to?

None whatsoever. The OPT winder has no way of knowing whether your design will include an even or odd number of gain stages. Therefore, no way to predict phasing at the primary.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 3rd April 2010, 07:50 PM   #10
diyAudio Member
 
Ty_Bower's Avatar
 
Join Date: Mar 2007
Location: Newark, DE
Quote:
Originally Posted by Svein_B View Post
...Feedback can be global, or local anode-to-grid as in the RH84 amp (described in other threads).
I've always been intrigued by the RH84 amp, and hope to build one some day. There's a few threads on it around here:

Building RH84 SE tube amp.
  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
EF95-EL84 single end integrated amplifier Musik007 Twin Audio 40 23rd October 2010 03:26 AM
6BQ5\EL84 line amp amp_guy Tubes / Valves 5 3rd February 2010 05:26 PM
6bq5/el84 yanamps Tubes / Valves 4 25th August 2002 09:53 PM


New To Site? Need Help?

All times are GMT. The time now is 02:59 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