Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Instruments and Amps Everything that makes music, Especially including instrument amps.

2-sided board for Ruby Amp
2-sided board for Ruby Amp
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 21st May 2019, 06:15 PM   #41
silasmellor is offline silasmellor  Denmark
diyAudio Member
 
Join Date: Nov 2016
Location: Denmark
2-sided board for Ruby Amp
Nice one Gnobuddy, though we have 230V AC here, but i guess in keeping with the spirit of such a project i might be able to find a transformer to step that down to around 120V. Looks like the 12C5 is a little tricky to come by though, but that might just be a matter of scouring the interweb.

On another note, Jaycar.co.uk has this line transformer: 5W Line Speaker Transformer | Jaycar UK Site, which supports 0.5W (this comes to 20K primary impedance as you mentioned). It only goes to 5W though, any tubes out there that fit that power level in Push pull? I don't need much to be honest, Ruby is plenty loud for most of my use and thats closer to 1W, if even that...
  Reply With Quote
Old 21st May 2019, 10:03 PM   #42
Gnobuddy is offline Gnobuddy  Canada
diyAudio Member
 
Join Date: Mar 2016
Quote:
Originally Posted by silasmellor View Post
...we have 230V AC here...
There are relatively inexpensive 230:115 and 115:230 mains isolation transformers available. If you want, you could probably use one of those.

As an example, here is the Triad N68-X ( N-68X Triad Magnetics | Mouser Canada ). It has two windings on the primary side, which you can wire in series for 230V AC mains, and in parallel for 115V AC mains. Either way, you get nominally 115V AC out of the secondary.
Quote:
Originally Posted by silasmellor View Post
Looks like the 12C5 is a little tricky to come by
I was a bit worried about that. I also think the All American Five radios were a North American phenomenon, with their deadly-dangerous wiring (direct to AC mains, with no isolation transformer ).

But here is where you need someone with a longer history with valves than I have - I know next to nothing about the history of European valves. But I would think there might be some at least similar valves there, perhaps used in radios or TVs, that will run on rather low B+, and cope with relatively low impedance loads.
Quote:
Originally Posted by silasmellor View Post
5W Line Speaker Transformer | Jaycar UK Site, which supports 0.5W (this comes to 20K primary impedance as you mentioned).
I found it very interesting that the Jaycar page actually mentions the primary impedance, and doesn't just list the power taps. I've never seen that for similar transformers sold in North America.

At any rate, this solves the difficulty of finding 12C5s! Using a Zaa=20k transformer immediately drops the AC anode current to one-sixth as much as you'd have with a 3.3k OT. You no longer need to have valves that can spit out lots of current at low B+.

The increased Zaa also drops power six-fold for the same B+ voltage. This means that as long as you don't go nuts with the supply voltage, your < 5W requirement is also easily met.
Quote:
Originally Posted by silasmellor View Post
Ruby is plenty loud for most of my use and thats closer to 1W, if even that...
Agreed. The last valve guitar amp I built spit out a mighty 200 mW or so - barely enough to heat up a quarter-watt resistor! And it is quite loud enough for apartment use, as long as it's connected to an actual guitar speaker.

On the Aussie Guitar Gearheads Forum, a prolifically creative member by the name of Darryl Hoy posted a 2-watt push-pull valve guitar amplifier using a pair of 6DX8 valves. This would have been about ten years ago, but Google might turn up some more details.

I designed and built a little push-pull guitar amp myself a few years ago, using a pair of 6AK6 pentodes, a 22.5k output transformer, and around 225V B+. It puts out somewhere around 2 or 3 watts.

More generally, the output power from a push-pull valve amp is roughly Po = 2*(B+ - 50)^2 / Zaa, where Po is in watts, B+ in volts, and Zaa in ohms. The "50" is an approximation for the voltage lost in the output pentode even when it is biased to maximum current.

So, given your possible 20,000 ohm (Zaa) transformer, you can calculate an approximate B+ voltage you need to deliver, say 2 watts output power. You can then use just about any pair of pentodes, and you'll get roughly the power you wanted.

Of course, if you use a pair of 6L6 or KT88, you'd be wasting most of their power potential (and a ton of cash). So it would make a lot more sense to use some little pentodes designed to drive the loudspeaker in a TV or radio.

-Gnobuddy
  Reply With Quote
Old 22nd May 2019, 01:35 AM   #43
thoglette is offline thoglette  Australia
diyAudio Member
 
thoglette's Avatar
 
Join Date: Dec 2008
Quote:
Originally Posted by Gnobuddy View Post
On the Aussie Guitar Gearheads Forum, a prolifically creative member by the name of Darryl Hoy posted a 2-watt push-pull valve guitar amplifier using a pair of 6DX8 valves. This would have been about ten years ago, but Google might turn up some more details.
That'd be this thread, for those playing at home.

Speaking of major contributors, Rob Robinette has also been getting into the low(ish) power amp thing, with a range of PP and SE 12AU7 variations on classic Fender (and related) amps. Look for the word "Micro" on his amp page
  Reply With Quote
Old 22nd May 2019, 06:04 AM   #44
Gnobuddy is offline Gnobuddy  Canada
diyAudio Member
 
Join Date: Mar 2016
Quote:
Originally Posted by thoglette View Post
That'd be this thread, for those playing at home.
Thanks for the link! Google didn't turn it up, and as I accidentally lost my login to my AGGH account some years ago, I couldn't use the forum's own search funciton.
Quote:
Originally Posted by thoglette View Post
Rob Robinette...low(ish) power...PP and SE 12AU7 variations
I found a few You Tube clips, and some of those amps didn't sound very good to me.

I wonder if using output triodes (instead of pentodes) has anything to do with it - I've read claims that output triodes distort too softly and progressively to sound right in a guitar amp. But I have no personal experience on this front, so this is essentially just hearsay.

I should add the caveat that most of the demos I found were made by people with negligible guitar-playing skills, which makes it harder to tell what the amp itself sounds like.

IMO that's a curse that plagues people who design DIY guitar amps - the typical builder is likely to have far better skills with his soldering-iron than with his guitar-pick!

-Gnobuddy
  Reply With Quote
Old 23rd May 2019, 01:29 AM   #45
thoglette is offline thoglette  Australia
diyAudio Member
 
thoglette's Avatar
 
Join Date: Dec 2008
Quote:
Originally Posted by Gnobuddy View Post
I wonder if using output triodes (instead of pentodes) has anything to do with it - I've read claims that output triodes distort too softly and progressively to sound right in a guitar amp. But I have no personal experience on this front, so this is essentially just hearsay.
Based on the discussions herein (particularly the effect of rail sag on screen voltage and the "knee" ) I'd say so. But I'm in a similar spot.

The next amp will have a convertible PP AB output stage (similar to Duncan's Blues 112) I haven't looked but I imagine that there's some youtube clips of that amp out there somewhere. Subject to the caveats you list
  Reply With Quote
Old 24th May 2019, 11:54 PM   #46
djgibson51 is offline djgibson51  New Zealand
diyAudio Member
 
Join Date: May 2010
Location: Hawkes Bay
Push-pull 6BL8 with a 5 watt line speaker trafo, powered by 12vac wall-wart into 240v/12v (reversed).
2 triodes preamp, FET concertina, pp pentodes out. Quite easily done
  Reply With Quote
Old 26th May 2019, 08:39 PM   #47
Gnobuddy is offline Gnobuddy  Canada
diyAudio Member
 
Join Date: Mar 2016
Quote:
Originally Posted by djgibson51 View Post
Push-pull 6BL8...
I hadn't explored this possibility before, so I plotted a few load-lines to see what they looked like.

The 6BL8 is generously rated for max. anode voltage, but max. anode current is quite limited, to only about 22mA at 30 volts anode voltage (the top left corner of the curves.) High voltage and low current requires a high load impedance.

On paper, then, a pair of these wants an OT with a 40k anode-to-anode primary impedance (see attached image.) Which is very high indeed, and I've never found an off-the-shelf transformer that can get this high.

Note that 40k Zaa translates to each pentode "seeing" a load of 20k for small signal excursions when both pentodes are operating, and seeing a load of 10k for bigger signal swings, when the other pentode has cut off.

If one could obtain this very special OT, then a pair of 6BL8 are estimated to produce about 1.8 watts RMS of output power, running on a B+ of 250 volts, and biased to about 5.5 mA idle current for each output valve.

In practice? Evidently people have managed to make these valves work with available (i.e. much lower) OT primary impedances. I couldn't find any load lines that looked reasonable on paper, but reality trumps on-paper estimates.

I would imagine the associated penalty would be much lower maximum power output (which might not be a penalty, if 1.8 watts is louder than desired.)

The last valve guitar amp I put together (on a bread-board) used an even smaller triode-pentode valve, a 6JW8. Single ended, it worked with a 22k reverb transformer and put out about 200 mW (which was plenty loud enough for me for use in my apartment.) Push-pull would have required an OT with roughly 45k Zaa, which I don't have, so I didn't try it.


-Gnobuddy
Attached Images
File Type: png 6BL8_Curves_Lines_005.png (137.2 KB, 13 views)

Last edited by Gnobuddy; 26th May 2019 at 08:50 PM. Reason: Point out that 40k Zaa translates to 20k and 10k at each anode
  Reply With Quote
Old 26th May 2019, 09:15 PM   #48
Gnobuddy is offline Gnobuddy  Canada
diyAudio Member
 
Join Date: Mar 2016
I found something (barely) workable using a 20k OT and push-pull 6BL8 pair, but most of the tubes operating area is left unused, overdrive tone is suspect (because the load line passes above the "knee" at top left), and output power is drastically reduced to about half a watt.

B+ is 170 volts, which is conveniently obtained from a 1:1 mains isolation transformer and silicon diode rectifiers in North America. Each pentode is biased to 7 mA anode current.


-Gnobuddy
Attached Images
File Type: png 6BL8_Curves_006_20k_Zaa.png (135.7 KB, 14 views)
  Reply With Quote
Old 27th May 2019, 01:40 AM   #49
djgibson51 is offline djgibson51  New Zealand
diyAudio Member
 
Join Date: May 2010
Location: Hawkes Bay
Thanks Gnobuddy. I got my 6BL8 tubes free from a garage clean out, and my 100volt line trafo for 12.5 cents (10 used for $1.25 on eBay)
And it sounds loud thro' a 12" guitar speaker.
Unusually, my 5w transformers have a .33 watt tap - boosting impedance to 32k(?)

I also have a SE 6BL8 bedroom amp using the same transformer without any apparent current saturation problems. And it has great overdrive tones - either directly into a speaker cabinet, or thro' the simple line out as a pedal before a big amp. This one is a keeper.
  Reply With Quote
Old 27th May 2019, 04:27 AM   #50
Gnobuddy is offline Gnobuddy  Canada
diyAudio Member
 
Join Date: Mar 2016
Quote:
Originally Posted by djgibson51 View Post
...it sounds loud thro' a 12" guitar speaker.
I have no trouble believing you on this. Using the stock 10" speaker in my '65 Princeton Reverb reissue, my 200 mW single-ended amp, set to clean tone, sounds loud enough for apartment use. Turn the gain up until it overdrives, and it is actually a bit too loud - I start worrying about bothering my neighbours!

Quote:
Originally Posted by djgibson51 View Post
Unusually, my 5w transformers have a .33 watt tap - boosting impedance to 32k(?)
Nice! And you also have the advantage of a 100 V primary. (It's 70.7 V in North America, which means a 0.33 W tap here corresponds to a smaller step-down ratio in the transformer, and a reduced primary impedance as a result.)

Quote:
Originally Posted by djgibson51 View Post
I also have a SE 6BL8 bedroom amp using the same transformer
SE needs only half as much transformer primary impedance, so your 30.3k should work quite well in SE mode as far as impedance goes. My earlier estimate was that even 20k was workable for SE, though 30k would let you use more B+ voltage if you wanted to.

Congratulations on how well the bedroom amp turned out!


-Gnobuddy
  Reply With Quote

Reply


2-sided board for Ruby AmpHide 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
Double sided PCB board with continuous traces? ErikdeBest Parts 6 27th June 2016 06:29 AM
Car audio LM3886 + SMPS on single sided board rwaudio Chip Amps 43 20th February 2015 02:12 PM
Single sided lm3886 board, opinions? Adam Eng Chip Amps 5 26th March 2014 02:18 PM
Ruby Tuby at 24V? pdavis68 Tubes / Valves 0 8th December 2013 09:23 PM
should i use a double sided board and mirrior the same circuit on both sides? Shaun Perez Pass Labs 12 10th November 2002 12:17 AM


New To Site? Need Help?

All times are GMT. The time now is 07:50 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki