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Latest Threads
Forum: The Lounge
Last Post: dalepsmith
10-04-2019, 11:19 AM
» Replies: 2
» Views: 116
20W SE poweramp
Forum: Core Poweramps
Last Post: Mike Knauf
03-11-2019, 02:32 PM
» Replies: 2
» Views: 82
Will this work? Output tr...
Forum: Homebrew Discussions
Last Post: trobbins
03-09-2019, 10:22 PM
» Replies: 3
» Views: 48
Generic 2 stage preamp PC...
Forum: Homebrew Discussions
Last Post: tristanc
03-06-2019, 12:01 PM
» Replies: 5
» Views: 37
Cliff Chappell 4-4-0 Heyb...
Forum: 4-4-0 Project
Last Post: headspaz
03-03-2019, 07:42 PM
» Replies: 3
» Views: 53
Hum in MTI SVT power amp.
Forum: Bass Projects
Last Post: Mark Abbott
03-02-2019, 01:56 AM
» Replies: 0
» Views: 19
Cheap SV83/EL84 Equivalen...
Forum: Homebrew Discussions
Last Post: Douche Baggins
02-28-2019, 10:02 AM
» Replies: 1
» Views: 21
Forum: Homebrew Discussions
Last Post: Douche Baggins
02-28-2019, 08:32 AM
» Replies: 1
» Views: 11
Humbucking Transformers
Forum: Homebrew Discussions
Last Post: Merlin
02-27-2019, 09:16 AM
» Replies: 2
» Views: 18
Bogen CHB35A
Forum: Homebrew Discussions
Last Post: MAOpedals
02-27-2019, 04:01 AM
» Replies: 0
» Views: 14

  20W SE poweramp
Posted by: Kraatz - 03-11-2019, 01:55 PM - Forum: Core Poweramps - Replies (2)

I'm a long time lurker first time posting. I have built a few pedals and a PCB version Firefly but my experience and knowledge are fairly limited.
I'm planning to build a low wattage Sunn model T clone, my plan is copy the original preamp and use a power amp from one of the projects here and I have a couple of questions.

Can I take the power amp for the SEL plans and swap the KT88 for a 6550 safely without changing any components? How about hooking it up ultralinear like the original model T?

Do you think the character of the amp would be better captured with a smaller push pull UL power amp with different tubes than the original or a SE UL power amp with the correct tube?

Thank you

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  Hum in MTI SVT power amp.
Posted by: Mark Abbott - 03-02-2019, 01:56 AM - Forum: Bass Projects - No Replies


My brother has a made in Japan SVT and it is for the most part a power house of an amp. I recently checked the bias and thought the amp has a lot of hum. I worked out most of the hum is in power amp stage. To be specific the hum is being largely generated by the first triode.


Here is a link which shows what I've already tried. There are pictures and waveform displayed.

Thanks for reading my thread.

Kind regards


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  Cliff Chappell 4-4-0 Heyboer O/P TXER
Posted by: Mark Abbott - 03-02-2019, 01:15 AM - Forum: 4-4-0 Project - Replies (3)


Regrettably Cliff Chappell passed away in Nov/2017, and I'm unable to purchase an output transformer from him.

Does anyone know the Heyboer model number for the 4-4-0 output transformer that Cliff had built for his Chappell amp kits?

I have checked various links and I come to a dead end every time.

Thanks for your assistance.

Kind regards


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  Cheap SV83/EL84 Equivalents
Posted by: Douche Baggins - 02-28-2019, 08:45 AM - Forum: Homebrew Discussions - Replies (1)

Just a heads up for anyone looking for cheap EL84 alternative. I purchased russian equivalent 6P15P that work and sound very well. The tube data sheet shows max 200v for G2 screen grid but I'm currently running 300v on them without any issue. I've read scant posts in other forums that people have ran them without issue on higher voltages as well. Purchased them from a russian seller for $3 each + $6 shipping. They are a direct replacement for EL84 wiring so no modifications needed.

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  Will this work? Output transformer "repurpose"
Posted by: MAOpedals - 02-28-2019, 03:12 AM - Forum: Homebrew Discussions - Replies (3)

I've got a Bogen CHB-35A with fixed bias 7868 power tubes.

I'd like to reconfigure the PA to cathode bias. First, I'll drop the plate voltage down to 450V. Then I'll need to deal with the output impedance. According to the 7868 datasheet, a cathode biased pair of 7868s at 450v have a load resistance of 10,000.


Here is what I have calculated for the Bogen's output transformer.

The output transformer measures as follows:
With 7.16vac applied across the primary, I get 0.184vac at the 4ohm tap.

7.16 / 0.184 = 38.8 turns. 38.8 * 38.8 * 4 = 6,039 primary impedance.

If I connect an 8 ohm speaker to the 4 ohm tap, the primary impedance would then be 38.8 * 38.8 * 8 = 12,128

To get a 10,000 impedance with this transformer, I would need a speaker load of 6.64 ohms. (38.8 * 38.8 * 6.64 = 10k)

And if all that is both correct and safe, can I then place a 39R 10W resistor in parallel with the 8 ohm speaker to get the 6.64 ohm load and the calculated 10,000 impedance?

I'm also calculating about 17% of the output being dropped across the paralleled 39R, less than 5 Watts

Is this math correct and will this work without frying anything?

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  Bogen CHB35A
Posted by: MAOpedals - 02-27-2019, 04:01 AM - Forum: Homebrew Discussions - No Replies

I've had a Bogen CHB35A stripped down and stored away for some 20 years. I'm finally getting around to rebuilding it. Looking for conversion suggestions.


First the hardware:
The primary transformer seems fine, puts out 259vdc with a bridge rectifier, 22uf, 440k bleed resister and no load. Also puts out 7.21vac with no load.

Didn't want to risk trying out the voltage doubling circuit with the original caps.

The output transformer's resistance from the center tap to plate is way off, 136R vs 198R.

But when I put 7.21vac through the secondaries, I get exactly the same voltage on either side of the center tap. 131vac @ 4ohms, 95vac @ 8 ohms, and 65vac @ 16ohms.

Does this indicate the turns are equal and may be ok?

I then put 7.21vac through the primary, got 0.259vac on the 8 ohm secondary. 7.21 / 0.259 = 27.73 (turns?) 27.73*27.73*8= 6151 primary impedance @ 8 ohms. Seems low.

Is this output transformer viable even though the center tap resistances are so far off?

7868 power tubes are not yet tested, but look fine.

Assuming the output transformer is ok, what to do with this thing?

I was considering the following: (with the voltage doubling circuit B+ = 490V)
1. A 2204-ish Bogen http://home.westman.wave.ca/~vwtweaked/b...0%20jp.jpg

2. An Ampeg-ish Gemini II https://tubeamplifierparts.com/schematic...ematic.gif

3. Or if it turns out the 7868s are no good, maybe replacing those sockets and doing an AX84 thing without the voltage doubler? (B+ ~250v)

I had been leaning towards the 2204, but the Gemini with some kind of tremolo is enticing.

Hoping someone out there has already been down this road and can advise.

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  Humbucking Transformers
Posted by: Jonno White - 02-26-2019, 08:26 PM - Forum: Homebrew Discussions - Replies (2)


I've got access to some EI laminations, but the stampings are too small for the power I need, so I was thinking of winding two small transformers with primaries connected in parallel and secondaries in series to get double the power output.

While musing over this, it occurred to me that their might be some merit in reverse winding one of the transformers and positioning so that they hum-buck the stray magnetic field?

Is this something that anyone has tried with any success?

It's probably not going to make a significant difference, and I would expect that any benefit might be outweighed by reduced regulation of the output, but thought I'd ask to see what peoples thoughts were.



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  Cascode Characteristics
Posted by: John Smart - 02-26-2019, 08:06 PM - Forum: Homebrew Discussions - No Replies

So I've been digging the grid leak biased cascode, like, a lot. I understand that the voltages on the upper grid and cathode/lower plate are whatever they are at idle, and will sag down to the design values under full overdrive, but I'm having a hard time understanding what defines the design values, given that they are dependent upon the upper grid voltage which is now variable.

Like, what determines where the voltages will sag to under full overdrive?

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  Someone want to host AX84?
Posted by: Chris Hurley - 02-25-2019, 07:19 PM - Forum: Site and System Discussions - No Replies

I thought I had this solved about a year ago but the plans fell through, so here we are again.

I'd really like to find someone to take over the AX84 project and BBS. It needs a rebirth and a new home. I'd like to find someone who is as passionate about it as I used to be twenty years ago and not just someone that wants to give it a warm grave.

If anyone out there is interested in taking over the reigns, please drop me a line. Unless you are pretty competent with webhosting technologies- PHP, etc..., its probably not going to be your thing.

The transition would look something like this:

* You will come up with a name for AX84's rebirth and we make an announcement with a timeline.
* You write translation software to convert data from very basic database structure to whatever forum you are planning to use. The AX84 BBS software will die in this process but I'll give you a sanitized database dump to use for the new site. People will need to re-apply for accounts because the passwords are hashed in a way that you won't be able to re-use.
* We will figure out a transition plan for the project files to move to their new home. The AX84 site software will die in this process.
* We will come up with an agreement giving you any rights that I have to use the project files for noncommercial purposes. I will also work out a favorable agreement to use the project files for commercial purposes that I have the rights to.
* We will link to the new site from AX84.com for a period of time.
* AX84.com will remain my domain name.
* I will ride off into the sunset.

Are you the one? Email me. I'm chrish.

Best regards,


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  Can i build a two channels hi octane?
Posted by: Bochi - 02-24-2019, 01:09 AM - Forum: Hi-Octane Project - Replies (2)

Hi everyone, this is my first post, i have an old stereo tube radio who has 12au7 and el84, i want to use it to build a hi octane, can i build two of them and feed it with the same power transformer and use it like two channels single ended? My idea is to build it with a 2 x 12 speakers, each of them in a different output transformer, and maybe put two standby switch to power off one of them when i don't need to much power. Is that possible?

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