Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on January 31, 2026, 04:27:54 am

Title: IS there a reason to build another AMP
Post by: TIMBO on January 31, 2026, 04:27:54 am: HELL YEAH !!!!!
After seeing this post https://www.aggh.net/discussion/index.php?topic=55279.0
And having a cab and chassis under construction, a circuit chosen from a well known amp builder
https://www.bing.com/videos/riverview/relatedvideo?q=jim+kelly+facs&mid=A2CE73DACDB09905C4F8A2CE73DACDB09905C4F8&FORM=VIRE
I believe this circuit is proven
(https://i.imgur.com/0m6yTDb.jpg)
And I've deleted the second channel as it is the same circuit for both channels, foot switchable between channels so you can have the second channel cranked for an overdriven effect
The reverb has been deleted for the moment
So a deep dive into my junk box and a couple of these were found
(https://i.imgur.com/ZDNX6Xp.jpg)
I found something similar on ebay in a 30 turret strip
(https://i.imgur.com/tFqWMDI.jpg)
The step drill did a nice job of the holes
(https://i.imgur.com/dJhMZyw.jpg)
(https://i.imgur.com/XTpan4t.jpg)
Even the socket holes line up
(https://i.imgur.com/pCTGg94.jpg)
A quick doddle to see how it would layout
Title: Re: IS there a reason to build another AMP
Post by: tubeswell on January 31, 2026, 11:09:35 am: There’ll always be a reason
Title: Re: IS there a reason to build another AMP
Post by: bmccowan on January 31, 2026, 12:41:25 pm: Yup - for me, if I stop for awhile I really miss it. And that's all the reason I need.
Schematic looks interesting - a bit of gain in that one, eh?
Title: Re: IS there a reason to build another AMP
Post by: Banjan73 on January 31, 2026, 03:17:28 pm: For me:
Theres always a reason. I don't mind building amps thats already have been built. But then, tubeamps is not actually the newest invention. So, maybe some of my builds have been build somewhere in the world.
My approach is to make something that sounds nice, off course. But with perhaps with a twitch.
And it needs to be useful for something. Many famous amps are, for me, not the "thing" I want to use. So I maybe will build them, as a base, and then modify them to my own preferances...
Title: Re: IS there a reason to build another AMP
Post by: Banjan73 on January 31, 2026, 03:23:21 pm: Hmm.
Just had a look on that schematics.
I'm looking at this on my cell phone, and its a very bad picture. Anyway, one question:
Those diodes to grid on gaintage 2 is new for me. Whats the purpose there?
Title: Re: IS there a reason to build another AMP
Post by: TIMBO on January 31, 2026, 04:07:42 pm: Apologies for the crap schematic photo, my printer ran out of ink and they want $65 for a cartridge
The diodes are 1N914's, I'm not tech enough to explain their purpose. :help:
Title: Re: IS there a reason to build another AMP
Post by: Rontone on February 01, 2026, 07:29:07 am: Quote from: TIMBO on January 31, 2026, 04:07:42 pm
my printer ran out of ink and they want $65 for a cartridge

check eBay, there should be some good self refill kits with syringes for a lot cheaper, do it outside though, it can get messy real quick!
Title: Re: IS there a reason to build another AMP
Post by: dogburn on February 01, 2026, 09:31:31 am: I was wondering about the diodes, too - they are also on V2. They are above my level of understanding as well.

Edit to add: Would they be clipping diodes?
Title: Re: IS there a reason to build another AMP
Post by: TIMBO on February 22, 2026, 04:20:20 am: Hey guys,
A bit progress in the wiring dept.
(https://i.imgur.com/CRRPdsn.jpg)
(https://i.imgur.com/sbbFEhF.jpg)
Title: Re: IS there a reason to build another AMP
Post by: Beezerboy on February 23, 2026, 05:58:18 pm: the diodes gotta be for suppression. they normally forward bias at about .7 volts so it looks to me like any negative voltage above 1.4 would be shunted to ground.
Title: Re: IS there a reason to build another AMP
Post by: glass54 on February 23, 2026, 06:17:16 pm: Its called "Clamping"
Starts when signal to diodes approach 0.6V (1.2V in Timbo's sch above) BUT asymmetric ie only works on negative half cycle.
I have seen where Marshall use diodes in Symmetric and Bi-direction format, thus clipping.
(Sometimes with a resistor in series for a "soft" clip)
Regards
Mirek
Title: Re: IS there a reason to build another AMP
Post by: Beezerboy on February 23, 2026, 09:34:52 pm: yeah.... thats the word I was trying to think of
Title: Re: IS there a reason to build another AMP
Post by: passaloutre on February 24, 2026, 09:48:06 am: Tell me more about that power supply. Looks like some kind of DC boost converter, but you have the output going to your diode rectifier?
Title: Re: IS there a reason to build another AMP
Post by: TIMBO on February 25, 2026, 12:05:09 am: Hey mate, I was having a rant about how expensive PT's are here in Australia and our friend Franco put me onto these
250W Vacuum Tube Amplifier Machine Switching Power 300V 0.6A 12.6V 4A 6.3V 4A 110V 220V AC TO DC Switch Power Voltage Conversion - AliExpress 1420 (https://www.aliexpress.com/item/1005005930993009.html?spm=a2g0o.tesla.0.0.72d58MJs8MJshp&pdp_npi=6%40dis%21AUD%21AU%24124.16%21AU%2448.11%21%21%21%21%21%402101ee6617632830244878432ee783%2112000034901120234%21btfpre%21%21%21%211%210%21&afTraceInfo=1005005930993009__pc__c_ppc_item_bridge_pc_same_wf__R52TZnP__1763283024724)
At $65 delivered I couldn't refuse.
Keen to try it out
Title: Re: IS there a reason to build another AMP
Post by: TIMBO on February 25, 2026, 02:43:54 am: Quote from: passaloutre on February 24, 2026, 09:48:06 am
Tell me more about that power supply. Looks like some kind of DC boost converter, but you have the output going to your diode rectifier?
Thanks for the prompt...
The unit came with no instructions and I was thinking the output voltages were AC
Confirmed all output voltages are DC
SO....
I'm thinking the HT supply is good to be connected directly to the standby switch of the HT supply with no extra filtering required at node "A"???
The 6.3v heater supply being DC can't have a artificial CT or connected to the power valve cathode to elevate.
Is it a floating circuit or grounded ???
(https://i.imgur.com/ByuEp30.jpg)
Title: Re: IS there a reason to build another AMP
Post by: glass54 on February 25, 2026, 04:54:09 am: Timbo
At the risk of being DAMNED ( :l2:) I would put a smaller Electro at Node A eg 10u 450 with 100nF 630V across it. This gives you a decoupled HT PS feeding your O/P Tx.
I would also put a 100nf 250V from Heater "0V' PS terminal to ground.
These suggestions are based on experience with Switchmode PS and DC heater P/S.
Looking Forward to see how it goes :headbang:
Has anyone seen a similar P/S Module with 400V out for HT?
Regards
Mirek
Title: Re: IS there a reason to build another AMP
Post by: Banjan73 on February 25, 2026, 06:11:49 am: Interresting PS module. How is it regarding noise? And are there any there any 3rd party safety testing stamps on it (I used to work at Nemko in Norway)?
Title: Re: IS there a reason to build another AMP
Post by: TIMBO on March 12, 2026, 04:46:02 am: A bit more progress...
(https://i.imgur.com/2Tw45Js.jpg)
Title: Re: IS there a reason to build another AMP
Post by: AX84CH on April 08, 2026, 08:44:42 am: I will be interested to read how your build turns out.

I recently built a small push-pull 6P3S amp using an SMPS. The one I got puts out about 280v. Dare I say "It works great" for the amp I built... enough to build another variation soon. I wouldn't mind 300v instead of 280v.