Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Car1 on April 19, 2018, 01:51:55 pm
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Hello all,
Very quick rundown, I am brand new member to this great tube amp forum however, I have enjoyed reading many posts for the last few months. So thanks to all who have handed down super great info and advice to those who have needed it. I have a few tube amps but I have never built one. Any relative skills that I have are related to my work in electric control panels for pumps and/or motor control.
Long and short I got a schematic and a layout, (JTM45 type) then headed to mouser, Doug Hoffman and classic tone for some supplies (I only bought a output tranny and choke from classic tone. No PT). The reason I didnt buy a PT was because I figured that one of the two I already have could be used? Both PT's were salvaged from old organs.
PT #1 350-0-350, a 13.2 winding with CT (CT to winding lead yields 6.3v), 5.1 rectifier winding, and lastly a 6.3v winding with a CT. However, at this winding the voltage outputs at one lead and CT to the other lead and CT are not equal. between the winding i get 6.3v but from the leads to CT I get 3.2v and 2.6v. I will be powering 3 - 12AX7's and 3 - EL34's. Question with this trans is with the 13.2V winding. Can I use that to heat the tubes? If so how should or could I drop the voltage down to 6.3v W/CT. Or is the 6.3v unbalanced winding the way to go?
PT #2 455-0-455, a 600v winding, a 5v winding, 6.7v winding, and a 35v winding. The only CT'ed winding is the HV. My heater winding needs a CT per the schemo and layout. I have heard of a Virtual CT but being so new to this I am reluctant to deviate from the schemo I have. Also the 455v is high for this build. The paper calls for a 690v.
Could anyone one here help me through this? Or advise me on what I should do? Also I do not have specs on these trannys. However I know for a fact they were taken from old organs. That being said (nobody chew me a new one here lol!) I am guessing that the rectifier, filament, and HV winding all flow enough current. These things weigh 9lbs 8oz a piece and were responsible for breathing life into many tubes. It would suck to not be able to use one of these great big perfectly functioning PT's
Thanks,
D :dontknow:
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Don't use PT#2. Voltage is too high.
PT#1 will also give higher voltage than I would want. Probably about 450 to 475 with tubes in. The Hammond JTM-45 replacement PT is rated 300-0-300 @ 230mA. EL34s will work fine with the higher B+, just be aware. A very common complaint is "How do I reduce my B+". The obvious answer is to choose the correct PT.
If you decide to use PT#1 (I would if I had it already), then use the 6.3V winding for the filaments. Tape off the CT and use an artificial CT. Very simple and easy to do. It's just two 100Ω 1/2W resistors connected from each side of the filament winding to chassis ground. One easy way to do this is place the resistors directly on the first EL34 socket. Connect one resistor between pin 2 and 8. Connect the other resistor between pin 7 and 8. BTW, there should only be two EL34s in that amp.
Would you share your layout and schematic? They will be very useful when/if you have specific questions about this build.
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Ok so I will use the 6.3v winding in PT #1, (Not one of the 12.6v winding leads and its CT?)Tape off its center tap and create an VCT with resistors. Couldn't I just use the 6.3v winding and the CT it already has? or because it isn't balanced close enough it cannot be used? OK I feel alot better that this can be done. And yes I can absolutely share the schematic and layout. Its just a kind of paint by numbers build. This is the only way I can go at it at this time. Need to get my feet wet first. Its a design by rob hull so all thanks to him.
BTW I just realized I mentioned 3 el34's! While I meant to say 2 el34's and the GZ34 rect tube (which I now see pulls a fair amount of current)
Thanks Sluckey
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Here is the schematic
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The layout file wont go through, too large but its from rob hull also. So I have to give credit to him for that also.
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Ok so I connect one resistor between pins 2 and 8 and then another resistor between 7 and 8? that's it? then twist my filament wire and daisy chain the rest of the tubes? Will this work when I get to the 12AX7 tubes which need more filament voltages I don't have to solder anything to the chassis? Pin 8 is the cathode, (which I know Sluckey is aware of) I say that bc I myself dont fully understand yet however, I can follow directions but at this point I want to know why or how that works?
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After a few hours of research and reading I know get it. And I see how the resistors could go to either a chassis ground but to also the cathode. Thank you Sluckey!
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Don't use PT#2. Voltage is too high.
PT#1 will also give higher voltage than I would want. Probably about 450 to 475 with tubes in. The Hammond JTM-45 replacement PT is rated 300-0-300 @ 230mA. EL34s will work fine with the higher B+, just be aware. A very common complaint is "How do I reduce my B+". The obvious answer is to choose the correct PT.
If you decide to use PT#1 (I would if I had it already), then use the 6.3V winding for the filaments. Tape off the CT and use an artificial CT. Very simple and easy to do. It's just two 100Ω 1/2W resistors connected from each side of the filament winding to chassis ground. One easy way to do this is place the resistors directly on the first EL34 socket. Connect one resistor between pin 2 and 8. Connect the other resistor between pin 7 and 8. BTW, there should only be two EL34s in that amp.
Would you share your layout and schematic? They will be very useful when/if you have specific questions about this build.
Not 1000 ohm, Sluckey got carried away with zeros. Must have been thinking of making a deposit in the bank. You want 100 to 220 ohm max.
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Don't use PT#2. Voltage is too high.
PT#1 will also give higher voltage than I would want. Probably about 450 to 475 with tubes in. The Hammond JTM-45 replacement PT is rated 300-0-300 @ 230mA. EL34s will work fine with the higher B+, just be aware. A very common complaint is "How do I reduce my B+". The obvious answer is to choose the correct PT.
If you decide to use PT#1 (I would if I had it already), then use the 6.3V winding for the filaments. Tape off the CT and use an artificial CT. Very simple and easy to do. It's just two 100Ω 1/2W resistors connected from each side of the filament winding to chassis ground. One easy way to do this is place the resistors directly on the first EL34 socket. Connect one resistor between pin 2 and 8. Connect the other resistor between pin 7 and 8. BTW, there should only be two EL34s in that amp.
Would you share your layout and schematic? They will be very useful when/if you have specific questions about this build.
Not 1000 ohm, Sluckey got carried away with zeros. Must have been thinking of making a deposit in the bank. You want 100 to 220 ohm max.
Wipe the sleep outta your eyes and read it again buddy! :icon_biggrin:
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Ω
note the day of week :icon_biggrin:
me, 4th order Cherry Pie for dessert :icon_biggrin:
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> two 100Ω 1/2W resistors
One of those zeros has feet.
Also: two 1K is not the worst thing in the world, except with very marginal cathode leakage.
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:l2:
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alt+234 Ω
alt+230 µ
alt+236 ∞
alt+225 ß
alt+224 α
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Hey Gents,
My first amp is coming together fairly ok. I made my own chassis, made up the board and soldered all the components. I was getting ready to make up the virtual center tap just like Sluckey had explained to me. I noticed in the layout there was a 1 ohm 1 watt resistor on pin 8 and pin 1 (going to chassis groung) of both power tubes. Since I was planning on creating a VCT by soldering a 100 ohm 1/2 watt resistor to pins 2 and 8, then soldering another 100 ohm resistor to 7 and 8, would there be any issue with there being that 1 ohm resistor soldered between pins 1 and 8? Also the schemo calls for R3000 diode on pins 3 of the power tubes (protection diode?) I dont have any on hand however I do have enough 1N4007 diodes to finish the job along with one 3 amp 1000v diode. Would I be OK with substituting a 1000v diode for the 3000v diode? I guess I just do not have enough experience to know whether or not i would get a voltage spike high enough to warrant having that particular component. Please correct me if I am wrong!
Thanks guys
D
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creating a VCT by soldering a 100 ohm 1/2 watt resistor
You doing artificial CT for fil?
put the 1ohm's IN
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Yes...for heater fils but I didnt know if there being a resistor soldered at pin 8 and 1 going to ground if that would have any effect on the VCT set up?
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Would I be OK with substituting a 1000v diode for the 3000v diode?
No. You need the 3000v rating for proper protection. You can use three 1n4007 diodes in series to replace that one 3000V diode. That' a total of six diodes.
Or you can just not use any protection diodes. The original JTM45 didn't have any. I probably wouldn't use them either.
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Since I was planning on creating a VCT by soldering a 100 ohm 1/2 watt resistor to pins 2 and 8, then soldering another 100 ohm resistor to 7 and 8, would there be any issue with there being that 1 ohm resistor soldered between pins 1 and 8?
Just connect the 100Ω resistors between pin 2 and ground and between pin 7 and ground.
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Ok Thank you gents!
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Don't use PT#2. Voltage is too high.
PT#1 will also give higher voltage than I would want. Probably about 450 to 475 with tubes in. The Hammond JTM-45 replacement PT is rated 300-0-300 @ 230mA. EL34s will work fine with the higher B+, just be aware. A very common complaint is "How do I reduce my B+". The obvious answer is to choose the correct PT.
If you decide to use PT#1 (I would if I had it already), then use the 6.3V winding for the filaments. Tape off the CT and use an artificial CT. Very simple and easy to do. It's just two 100Ω 1/2W resistors connected from each side of the filament winding to chassis ground. One easy way to do this is place the resistors directly on the first EL34 socket. Connect one resistor between pin 2 and 8. Connect the other resistor between pin 7 and 8. BTW, there should only be two EL34s in that amp.
Would you share your layout and schematic? They will be very useful when/if you have specific questions about this build.
Not 1000 ohm, Sluckey got carried away with zeros. Must have been thinking of making a deposit in the bank. You want 100 to 220 ohm max.
Wipe the sleep outta your eyes and read it again buddy! :icon_biggrin:
He did some of that moderator cleanup. I saw 1000, but then again I do and I have seen thing that really aren't there. :l2:
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Hey Gents,
I finished my first amp. Its a JTM45 type clone. I'm super pumped, I learned a lot, and I'm getting ready to build another. It sounds great! Man its loud, punchy, and hits very hard. Being my first attempt at this I scavenged and/or made a few of the more expensive parts like the power transformer, chassis, rectifier tube and the pre-amp tubes. The GZ34 rectifier tube and the RCA 12AX7A tubes came off of an old organ along with the PT. All the wire, bus wire, grill clothe, knobs, amp handle, and other parts were scavenged. I made the head cab and upholstered it last night.
I want to thank all of the guys that jumped in and answered the technical questions I had about this project without which I may not have been as successful or as happy as I am right now with this amplifier. So a big thanks to Sluckey, Ed_Chambley, PRR, DummyLoad, and Shooter.
PS I ran out of knobs lol
D
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It's certainly a unique and different looking head.. nice work. :icon_biggrin: