Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Kev-W on January 08, 2024, 12:01:37 pm
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Hi, a new member joined to thank Steve Luckey for his drawings & design for this amp, I had a poor bought clone of a Vox Night Train (the fizz was cloned too) so wanted to repurpose the valves & trannies, and had built a couple of RobRob micros so have had a crack at this but with ss rectification & the MV
My B+ is a little high at 362vdc but I missed a pair of resistors on the old board before the rectifier, adding them dropped the voltage by 6v (they're 22ohm & the schematic shows 220o) so I've ordered a bigger pair. The cab's not quite finished but is made from oak flooring :) and the chassis is homemade.
But yeah, cheers from England fella for sharing your design for a great sounding amp!
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Glad you like it. There have been several built that I know of. It's really a lot of tone in a very small box.
I noticed your circuit board jig. I bought one of those about 6 months ago. Pretty neat and cheap too. I replaced the crossbar on mine with a longer wooden bar and can handle up to 22" long circuit boards. Easy to do if you're wood savy and have the tools. Take a look...
https://el34world.com/Forum/index.php?topic=30267.msg333898#msg333898
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What made you mount xformer like that, out of interest what make/model is it?
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What made you mount xformer like that, out of interest what make/model is it?
Good question, I don't know any better's an honest answer, and the chassis is from 1mm ali so I thought it may weaken the box too much, and the couple of micro's I've built have been ok that way, and the donor amp was laid out the same way (I forgot that but hence the edit 🙂)
It's an unknown make, 240vac primary, 250vac secondary with a ct & a pair of non ct heater wires, Chinese writing that a phone translator gives nothing useful from, it's quite heavy and did run the 2 el86's before so I took a gamble on it handling an extra preamp valve's heater. The donor amp was Chineseium & rebadged (stickered) with a western name, being cheap was its selling feature :icon_biggrin:
I should say I'm not a tech, more a meddler with engineering expeirence and happy to learn (or try to) as I go.
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Brillian work on the chassis and faceplates! Was about to go straight to asking where they came from as there is a dearth of amp parts suppliers in the UK, so I was equally disappointed and impressed seeing it was all homebrew :)
I'd say it's worth tinkering with the filter resistors to see how you like the differences, but 360v is in a very usable ballpark, and what many Marshall 18w and Vox AC15s run at, as well as all of the Matchless 15/18w amps which are extremely close to AC15 values. I'll be putting together a very similar amp soon and 360 is what I'm aiming for anyway!
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Brillian work on the chassis and faceplates! Was about to go straight to asking where they came from as there is a dearth of amp parts suppliers in the UK, so I was equally disappointed and impressed seeing it was all homebrew :)
I'd say it's worth tinkering with the filter resistors to see how you like the differences, but 360v is in a very usable ballpark, and what many Marshall 18w and Vox AC15s run at, as well as all of the Matchless 15/18w amps which are extremely close to AC15 values. I'll be putting together a very similar amp soon and 360 is what I'm aiming for anyway!
Thanks 🙂 the face plate was done on my diode laser (first attempt with plastic), a friend did me one for my micro Bassman last year, the 2 before that were done with vinyl Uncle Doug style, the chassis was my 2nd attempt as the computer side panel i first used didn’t work out deep enough for separate switch/light.
I’ve a choice of 60r & 82r resistors to swap for the 22r ones and will likely try the lower value ones first hoping I don’t have to swap any more to bring the preamp B+ back up, but it’s a learning curve.
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The hum went when I re-routed the V2 pin 9 wire under the board and reflowed the joint, chopsticking identified where the hum was, but I swapped the MV volume wires to shielded cable for good measure, I've 352vdc at B+,325vdc at B+b, 292vdc at B+c and 268vdc at B+d so all around 30v higher than the schematic, pin 7 is 349vdc, pin 311.8vdc & pin 9 324.2vdc on V4 & V5, and it sounds sweet, I'll pop a pic of the finished job when the cab's dried and I remember to make the front bottom panel.
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And lastish but by no means least is the finished amp, remarkably quiet for the volume it'll produce, I've a mate who's good on guitar and will give me some sound clips at some point but here's the photo's, the speaker cab has an 8ohm 10" 30 watt greenback in it and ironicly I made it for the amp I just butchered for this, it's what got me into meddling with amps I guess. :laugh:
The 'Acomb amplification bit's a mickey take of my side of town and only exists in my head and for a bit of humour.
And cheers for letting me share it with you.
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Looks great! This amp is the one I reach for first. I just like the sound of that simple Vox preamp whether it's plugged into a 2x12 Celestion heritage or a single blue.
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Looks great! This amp is the one I reach for first. I just like the sound of that simple Vox preamp whether it's plugged into a 2x12 Celestion heritage or a single blue.
It may be a 'simple' Vox pre amp to you but I found it reasonably complicated :l2: and it does sound like the Vox sound I'd hoped to hear which was the point in stripping a working amp to get and I'm glad I did, the Marshall channel is underused so far but it's great too, for me with this I feel I need a better quality rotary switch as there's a crackle/needs a tap sometimes switching tho I get the 6 tones, and that's a great feature in itself.
I need to avoid buying a pair of 2nd hand 16 ohm speakers as I've enough oak to do a double stack kind of thing and the neighbours aren't out enough to make it worthwhile :laugh:
I notice you've made an Uncle doug like reverb with drawings which may be next winters project.
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I notice you've made an Uncle doug like reverb with drawings which may be next winters project.
Are you referring to my Revibe?
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I notice you've made an Uncle doug like reverb with drawings which may be next winters project.
Are you referring to my Revibe?
Yes your Revibe, it's tempting as I've made 2 Deluxe micro's and I think I can use the transformers from the amp that's rarely used, I made 2 as the first one was my first amp, and I was so surprised it worked I made another...
I should have mentioned the ppimv, it works fine for taming the amp for home playing, previously the donor amp needed a power soak!
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Is a sound clip of the ef86 channel, my mate on a MiM Strat, worth a listen :icon_biggrin: