Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: rafe on January 24, 2026, 11:37:02 am
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Schematic depression searching my soul......Picked up a 420 off of e-bay I have the chassis on the bench, and it looks like I'm the first in there and it looks good. I am planning on the electrolytic caps filter and bypass(6caps) to be replaced' a 3-prong installed, the 2 .01 ac caps removed. is the neon lite safe where it is? ...I am working off the schematic where C supplies the plates on V2 PI then mysteriously turns into D (as it is clearer.) This amp is very much like the thunderbolt I have .....but it's been a long time since I was in there, It had the double (reversing polarity) but I don't remember 2 ac caps. I am going to test the amp as-is everything looks good and i am considering leaving the smaller caps as-is. Thanks for any insights you may offer, Rafe Harmony_h420.pdf (https://el34world.com/charts/Schematics/files/Harmony/Harmony_h420.pdf)
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Congrats on picking up a sleeper. The good news is you can buy them cheap. The bad news is that you need to sell them cheap. As you state - it's the SS rectified Thunderbolt in a different chassis and cabinet. I've rewired two of them and one Tbolt. There is also a Tbolt with a tube rectifier. Interestingly there are inaccurate Tbolt schematics out there, but the H420 schematic I used was spot on.
I left the AC lamp as it was - but I did not spend time thinking about that. I believe it's safe.
schematic where C supplies the plates on V2 PI then mysteriously turns into D
I think that the "D" should really be at the junction of those 2 100K resistors. I think you will find that the cap at that node is not a 4uf but is a 0.1 or a 0.01. The 4uf ? seems to trace back to one schematic. I kept the values of the filter caps the same, but used a 33uf in place of the parallel 20/10 caps. I re-wired the power switch as a single pole. Good discussion here: https://el34world.com/Forum/index.php?topic=26084.0 (https://el34world.com/Forum/index.php?topic=26084.0)
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I see a .1 there. I brought it up slow on the variac ....it came with a 20amp fuse, calls for a 3amp slow-blo I am out of them but have some 2 amp and that worked. checked voltages as I brought it up. no tubes at first all voltages were reasonable for no tubes. Put in a set of tubes I scavenged out of stash because the tubes in it were 1965 rca's 2 12ax7a's and 2 6l6gc I don't think they have much time on them and didn't want to risk them ....Voltages look very close to the specs with the tubes in none of the caps showed any leakage I ran it for an hour and checked again voltages good. Then I played it for an hour or so, really nice my shop speaker is a 2x12 closed cab.
Still need to do the 3 way Those .01 caps are coming off the fuse, I am wondering if they can stay? My thunderbolt is rectified I have the filter caps here and I will keep thinking about installing them as I clean the cabinet and grillcloth and play the amp. Nothing has been touched in the amp 61 years old and going strong, Outstanding!
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"I kept the values of the filter caps the same, but used a 33uf in place of the parallel 20/10 caps."
That was my thought too. Makes you wonder why they did that, probably had lots of 20's and 10's on hand.
My Dad bought me an H420 when I was 12 I was playing bass and it served me well for 3 years until I worked all summer and bought a brand new 68 drip edge bassman ....."Well, It took me years to get those souvenirs' And I don't know how they slipped away from me"
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That was my thought too. Makes you wonder why they did that, probably had lots of 20's and 10's on hand.
Agree - one of those is in the cap can, the other is an axial in the chassis. I suspect they didn't want to waste the cap cans when they realized they needed more filtering.
If it were mine, I would replace those old filter caps. You will not regret it.
"Souvenirs" One of my favorite John Prine songs.
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I actually have the caps and 3prong ready to go all the resistors but two were within spec. I replaced them......I'm doing a little each day, the tolex had a few small spots I had to glue down. The grill cloth is stained and I'm going to remove the 15 and see if I can get that out. All the voltages are spot on, except the cathodes on the 6l6's
the schematic calls for 37v and i have 27 I'm wondering if the schematic is wrong. the 200 ohm cathode resistor is spot on. Amp sounds great through the 212 cab. I really do like 15's though, so I am looking forward to that. I have a 20 a 10 and 2 16's ready to recap this amp, The terminal strip is a close fit but will be ok. I think I may have to modify the the ground mount to move it away from the power cord.
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the schematic calls for 37v and i have 27
Maybe the tubes just a bit weak?
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Good to know, that is very likely, they are well used Fender branded tubes out of a 64reissue I bought new 15 years ago I'd guess. I replaced them in VV with RCA's.They are running at 65+ m.amp I did go ahead and redo the filter caps. Still didn't do the 3 prong and still don't know if the .01 caaps should be hanging off the fuse end
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still don't know if the .01 caaps should be hanging off the fuse end
Do you have a pic of what you are seeing here? Are you referring to those two "death caps?" If so I would think you would remove those as you install the 3 prong power cord. "Safety" type X or Y capacitors are sometimes used on the AC input. There are some discussions of that on this Forum.
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I will go out now and take a photo, For some reason , I am having difficulty getting my mind around the switch set up. Normally I go with black to switch, switch to fuse tip, fuse to Trans former getting rid of the death caps. I have an extra terminal that is free and I think I should bring the common there with the second leg of the neon then to trans. I am pretty sure that's the way to do it here , I'm pretty sure my Thunderbolt has a neon AC light and I did it that way....so long ago
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I figured it out ,this will work
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First - you draw like me:laugh: what I call refrigerator art.
Yes - that works.
I need to sell the two that I have. I thought that maybe they'd go up in value as more folks realized they are Thunderbolts in sheep's clothing. But that doesn't seem to be happening.
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I do much better with pencil and paper :dontknow: Here is before 3prong and after.....I'm slowly bringing up caps as I write
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"I thought that maybe they'd go up in value as more folks realized they are Thunderbolts in sheep's clothing. But that doesn't seem to be happening."
Yeah, I was talking about that the other day it doesn't make sense. That's just an odd mindset to me. I would have thought the "420" alone would fetch a premium. I got it at a fair price and have a seller lined up if I want to recoup the dough, as of now I don't want to. I picked up a 64 Thunderbolt when they were pretty high. It was in an open barn and had a rat's nest behind the chassis.....I asked her what she wanted for it and 3 x's she said make me an offer.....got it for $60. cleaned it up recapped it and installed a 3prong, That was 2008 and my first recap and 3 prong Still one of my favorites
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would have thought the "420"
X
# of Musicians looking
/
bags of Cheetos
+
pi
and you have the mathematical equivalent for
making sense of humans :icon_biggrin:
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Well, I buttoned everything up earlier after testing everything brought it inside and was playing it for about 15 or so minutes when the volume dropped...... when I first fired it up I heard some strange hissing crackling but it went away, the amp sounded great then the 1/2 V1 became microphonic bordering on feedback for a few minutes and dropped out to a low volume, I looked and One of the 6l6's was redplating. I'm done for the day but I'll open it up tomorrow and see if I dropped a resistor. And check voltages again......The 15 sounded good .... :BangHead:
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Old amps are like old cars that haven't run in awhile, eh? Pour some gas in the carb, jump the battery, it coughs and it runs - for 15 minutes, or seconds, and then things start burnin' up. Oh well. unless a tranny was damaged, no big deal, you'll get it.
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The voltage in the house is 121 I had the variac on 117 in the shop ....in the shop the voltages were spot on except cathodes were 27 instead of 37 ....in the house the plates were 417v the grids were 412v close to 80 ma. on the 6l6's.( that's not good ) and likely the red plating. The tubes tested good no shorts....I'm going to bring in the variac and see how it does on 117....everything else looks good.....Not sure how to tame this for 121 vac
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Not sure how to tame this for 121 vac
A Buckaroo would be a nice/cheap companion for that amp. See page two of my amp scrapbook...
https://sluckeyamps.com/misc/Amp_Scrapbook.pdf
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OK, years ago I made a bias probe an old tube socket to a Eimco milleamp meter. I put it in the socket that red plated and I fired it up, played through it and it sounded great. the meter was @ 58ma.....after a few minutes it dropped to 50 then 40,30,20 and I turned it off.. no red plate.. something heating up and leaking????what is the likely culprit, cathode resister??? It sounded great up to the turn off
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"A Buckaroo would be a nice/cheap companion for that amp."
Yes sir and a lot of my other amps as well. I am Lucky you posted that :worthy1:
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I just got back to this today and have it out in the shop....Checked as much as I can and it still runs away when it has been on for a while. I ordered parts as I think a component or multiples are heating up and failing, I suspect the .05 and 47k's (drifted a little more than 10%) on the 6l6 screens. also going to replace the 35/50 electrlytics on V1 and V2 with 50/50's .....then give it another try....parts are coming :BangHead: :dontknow: :w2: :l2:
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"Oh well. unless a tranny was damaged, no big deal, you'll get it."
Replaced the aforementioned parts and replaced the tube that red plated ....but it still kept frustrating me, so I'd walk away from it for a few days and go back and prod and poke around. The last few times I played through it (all the voltages were spot on ) but something was wonky it would sound good but the volume would jump about slightly and subtly with guitar volume on 10, from 1-9 the guitar volume was not responding normally very low and would not increase very much until 9-10 .....I suspected the inputs and resistors as the guitar is known to be good so I prodded around that area.....I had the guitar strapped on and as I moved, I stepped on the cable and on the right spot too....It was a continuity problem with the AC signal from the guitar to the amp. This amp sounds really good now.......and should be ready for many more good years.....bmccowan if you get back inside yours let me know what the cathode voltage is on the 6l6's mine is still lower at 28.5 and after seeing mistakes on both schematics, I suspect the 37v was one as well