Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Shack on February 08, 2019, 09:39:19 am

Title: building a second 5f1 with a different power transformer (question)
Post by: Shack on February 08, 2019, 09:39:19 am: Hi all, been awhile since I have been on here, hope everyone is doing good. Anyway, I recently built a champ with the transformer 02277( 325-0-325 70 ma ) and the OT is 8k to 8 ohm. This amplifier sounds fantastic......so I want to build one for my other guitarist in the band. I have an old tranny I bought years ago, that has 250=0=250 at 70 ma, and I plan to use a GZ-34 rectifier instead of the 5Y3.

In order to get this amp as close to mine as possible, I should change the dropping resistor values correct? I ask because I bought this tranny to build the small single ended amp in the Dan Torres book but never did, and he used way lower resistor values in those plans, I just want to make sure before I place a parts order.

I hope this amp sounds alot like the one I just finished, because I am in love with the sound of that one.
Title: Re: building a second 5f1 with a different power transformer (question)
Post by: jjasilli on February 08, 2019, 10:31:36 am: 325 X 1.1 (5Y3) = 357.5 plate volts

250 X 1.3 (GZ34) = 325 plate volts

This is within about 10% and "shouldn't" make a difference in the power amp section. Probably not much difference in the preamp either, but. . .

Smaller B+ dropping resistor(s) will enable you to match your plate voltages in the preamp section. I would use Ohm's Law to calculate the preamp current draw establishing the preamp voltages in your amp. Then use that info to calculate the dropping R value in the new amp. (Though some here prefer guesstimation + trial & error).
Title: Re: building a second 5f1 with a different power transformer (question)
Post by: Shack on February 08, 2019, 01:00:19 pm: thank you, I need to write down all these formulas so I can figure out stuff like this without asking....so much math in electronics lol
Title: Re: building a second 5f1 with a different power transformer (question)
Post by: jjasilli on February 08, 2019, 01:35:28 pm: here's a rectifier info chart.

also Ohm's Law is a must (I feel your pain.)
Title: Re: building a second 5f1 with a different power transformer (question)
Post by: shooter on February 08, 2019, 03:44:46 pm: and here's the ohms law cheat-sheet
Title: Re: building a second 5f1 with a different power transformer (question)
Post by: Shack on February 10, 2019, 08:44:32 pm: so after a week or more of jamming this amp.....I played it with the band......and halfway through rehearsal, the sound went away. and I think the output transformer smoked on me, what would cause this, and where do I start to troubleshoot.? the amp appears to be built like a tank, and im about to start another, guess ill look at things tomorrrow, but it was working fine

My guess is that the old Sylvania 6V6 may have gone bad, because both transformers were hot when it went quiet, so when the new transformer comes in, ill fuse it ....its just going to set back my friends champ a week or so,lol
Title: Re: building a second 5f1 with a different power transformer (question)
Post by: jjasilli on February 11, 2019, 07:48:00 am: Always suspect tubes first. Then test the OT. If a tranny is bad, don't suspect the tranny itself was faulty. Instead check for other circuit faults - like a short circuit - that may have fried the tranny. Otherwise if a new tranny is installed, it will fry too!
Title: Re: building a second 5f1 with a different power transformer (question)
Post by: Shack on February 11, 2019, 03:28:59 pm: Yea, thats why I think that the old 6L6 is most likely the culprit, since it worked great and I so far cant see anything else to look suspect.