So I have a small output transformer to play with coming from an old battery radio. Effectively a 8K push pull transformer. (there is no existing documentation on the radio so its dead).
I offered my classmate a chance to have one tube guitar amp because when he first heard his guitar trough my hifi tube amplifier fell instantly in love with tube stuff.
(I know I wont be able to replicate it exactly to what my big amp sound like but its just the tubes he wants)
Since I wanted to do something he will be familiar with I have chose then Marshall JCM800 schematic.
Basically use the entire schematic as is except the output stage will be consisting of a push pull cathode biased EL84 output.
My only concern is the lack of voltage for the preamplifier and the not required crazy amounth of drive from the phase inverter driver section.
The estimated voltage wont be much more than 320V which is much less than the 470V for the EL34 stage. I am concerned about the preamplifier how will it affect the gain and other characteristics.
Would it be a big problem if I left the schematic completely stock no value change and just undervolt it and call it a day or I will have to change a few parts arround to make it perform properly?
(I am attaching the schematic with voltage measurements aswell. The amplifier I want to make will have a simple cathode bias EL84 output stage not the EL34s)
I offered my classmate a chance to have one tube guitar amp because when he first heard his guitar trough my hifi tube amplifier fell instantly in love with tube stuff.
(I know I wont be able to replicate it exactly to what my big amp sound like but its just the tubes he wants)
Since I wanted to do something he will be familiar with I have chose then Marshall JCM800 schematic.
Basically use the entire schematic as is except the output stage will be consisting of a push pull cathode biased EL84 output.
My only concern is the lack of voltage for the preamplifier and the not required crazy amounth of drive from the phase inverter driver section.
The estimated voltage wont be much more than 320V which is much less than the 470V for the EL34 stage. I am concerned about the preamplifier how will it affect the gain and other characteristics.
Would it be a big problem if I left the schematic completely stock no value change and just undervolt it and call it a day or I will have to change a few parts arround to make it perform properly?
(I am attaching the schematic with voltage measurements aswell. The amplifier I want to make will have a simple cathode bias EL84 output stage not the EL34s)
Attachments
The first 10K/2W power supply dropping resistor can become a Hammond 154E. With your decreased B+, the choke will offer less voltage drop but still plenty of power supply noise rejection. This won't completely make up the difference, but it's a start.
Traditionally guitar amps flog the snot out of EL84s, so going with 400V of B+ is inappropriately appropriate.
Traditionally guitar amps flog the snot out of EL84s, so going with 400V of B+ is inappropriately appropriate.
You can run the preamp voltages at a lower voltage. In fact, I have a Friedman BE100 here and is a hot-rodded JCM800. Voltage at the plate of the first tube is 169VDC. You will have to increase the feedback resistor as EL84 do have a lot more gain than EL34's. I have probably built more EL84 guitar amps than anyone fwiw.
It was indeed a vacuum tube radio based with very weird and hard to find tubes from tungsram(by that I mean very hard to find data about them).
Best I can figure it used a load of 1S5T (or IS5T i really dont have an idea) IT4T DLLI something something(cant really recognise the writing) and there were 4 of the same tubes (judging by the internal construction of the tubes and similarities in writings) suggesting a paralel push pull arrangement.
I couldnt find any documentation on the internet for this radio, best I can figure its a west german radio and to be honest I am not really looking forward to replace every cap that has turned into a insanely brittle fossil.
I actually thought about linear regulation for the preamp with a transistor and so on. Easy but effective. Yeah a choke would have been an easier way off but for 15 dollars I can do a lot better with transistor regulation.
Yeah I have seen people pushing EL34 grade performance on class B with those things. I would prefer to conserve the tubes at least a little bit. This aint the "money no objection" kind of project. Thats where my personal hifi amplifier excells.
Yeah I thought about it for a while and I have to say I have never seen this implemmentation of NFB (granted I did hifi amps only up to this point). Makes it just a matter of playing with a VR and finding the best value.
Well thanks for the feedback ill try it some day when the raw stock arrives and come back to here to post my findings
Well thanks for the feedback ill try it some day when the raw stock arrives and
come back to here to post my findings
You may need to experiment with the polarity of the feedback connection,
depending on how you wire the primary. Wrong way = oscillation.
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1S5T = 1S5 = DAF91
1T4T = 1T4 = DF91
Tungsram often put a »T« somewhere to their tubes' designators.
If there weren't any other tubes than these battery powered ones, I suspect that the OT you've ripped out might be too brittle for a EL84 design.
Best regards!
Thank you kind sir for the literature. I suspect you did not read the first post because I mentioned that the impedance of the transformer is 8K anode to anode (roughly).
I did not need to do math on the subject because I have an impedance tester and I just put a 4ohm load across the secondary and bam the meter shows roughly 8K (tested at 100Hz 1kHz and 10kHz)
I have the same concern on the matter. The DC resistance on the primary side was a bit high to my taste..
I dont really want insane power in fact 10W is more than enough because we will be using the original speaker (for vintage sake).
Dont the road it might get a toroidal output transformer but not any time soon.
I dont plan to run all that horrible idle currents on it aswell. I am considering grid biasing and tweak it with a scope to the exact point where I dont get crossover distortion.
Overall low B+ (320V) low current low power. I hope the OPT will survive.
Relocated to Instruments & Amps per forum policy.
I have a feeling that thanks to this, this thread does not get the same attention.. I was looking for technical advice on tube gear not general live audio. It appears more people go trough the tube section than live audio.
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You suspect wrong, I did read your first post.
You sounded like you had no idea on the tubes operation and were trying to find information. You did not say how you determined the transformer was 8k, you never said you are versed in electronics or that you have any equipment to use. We get some that show up with less than a multimeter and want to design amplifiers. Up to the point of my replying you did not show where your capabilities lie.
Just build a JMP preamp on a 18 Watt.
That's true, but what attention you do get, will come from people who understand that guitar amps are very differently engineered than Hi-Fi amps, and for good reasons.
The tube Hi-Fi forum has a lot more participants, but many of them have discarded logic, science, and engineering, in favour of subjective beliefs in "tube woo-woo", which often boils down to the belief that if you pay a lot more for a component, it will always sound better.
-Gnobuddy
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