Advice on constructing an 807 valve guitar amp

[nzoomed]

I have a few spare 807 tubes, and thought it would be a good option for me to build a valve amp using them, the only amps ive worked on in the past have used el34 or 6L6(which shares some similarity to the 807).

I want to improve on this design below, as there is no real first stage for this amp, other than the phase splitter itself. Ideally, i would like to have a 3 stage preamp, the first stage for a high and low input, then a second stage preamp, using both sides of a dual triode in parallel, before being fed into the phase splitter.

This design is using a 2c51 (5670), but i would prefer using 6sn7 tubes or ecc83 tubes, or a mixture of both, any advice there would be useful.

As you can see, its designed to switch between triode and tetrode modes.
I would like to get more wattage out of it if possible, but thats probably a big ask, even though i know you can get 80 watts out of a pair with 600v plate voltage, it may get more distortion and probably would be better to use a quad set of tubes if i was to go down that track, the power supply of course would need to be made bigger to allow a higher voltage, as its probably can only put out 500v max looking at the design.

I will have to design my own power supply anyway, but i do like its features with DC current for the preamp heater etc.

I expect a dual set of 5u4GB rectifiers should provide enough current as it stands at present, as the rectifiers used heare are expensive and hard to come by. I have a few 5u4 tubes available too.

I will want to add some extra controls, for bass, mid, treble, and presence and i will want to add a preamp volume control in addition to the master volume.

**broken link removed**

more info is on this link below, click on jp806:

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click on "jp806" to get the page

Thanks for any advice relating to this

 

[k7elp60]

I see no need of R9&R10, and you may have to adjust the value of R20 as the screen grid voltage of the 807's is a max of 300V.

 

[nzoomed]

i dont see any need either for R9 and R10.
i do agree with you about r20, thats probably why its left blank, so it can be adjusted accordingly, at 300v it possibly could cause distortion, but i will experiment up to 300v of course.

 

[nzoomed]

contd

i am thinking of modyifing the 1st, second and third stage of the preamp working from this design here.
phase splitter end:
https://www.ax84.com/static/corepoweramps/50W_PP/AX84_50W_PP_Poweramp_Schematic.pdf
preamp:
https://www.ax84.com/static/corepreamps/Lead/AX84_Lead_Preamp_Schematic.pdf

any advice on incorporating this would be great.

 

[JimB]

R9 and R10 are what is known as "grid stopper" resistors.
They are there to prevent high frequency parasitic oscillations and should be fitted close to the valveholder pin.

I find it rather strange that in an otherwise all "vacuum state" amplifier there are a couple of LED indicators on the LT and HT supplies.

JimB

 

[audioguru]

Where will you find an old output transformer?

 

[Nigel Goodwin]

Where will you find an old output transformer?

You can buy them quiite easily, but they are VERY, VERY, VERY expensive!!!!.

 

[nzoomed]

R9 and R10 are what is known as "grid stopper" resistors.
They are there to prevent high frequency parasitic oscillations and should be fitted close to the valveholder pin.

I find it rather strange that in an otherwise all "vacuum state" amplifier there are a couple of LED indicators on the LT and HT supplies.


JimB

Yes i agree with you, i will use a 6v lamp for the filaments and a neon globe for the main power instead.
i will have to heavily modify the power supply, i think i can get away with using one 5u4 tube or similar rectifier, but i may use a dual tube rectifier to deliver more current, i can get the transformers wound up no problem, so should be OK. Thanks for that info on the grid stop resistors, ill leave them in. If anyone can help me in the area of replacing the phase splitter with an ecc83 or 6sn7, that would be a great help, i will have to change a few values of the resistors.

 

[audioguru]

A high fidelity output transformer for an old tubes amplifier is completely different than a low frequency power transformer. The amplifier you are looking at has negative feedback that will cause an improperly-made audio transformer to have phase-shift which will cause the amplifier to oscillate. I hope your transformer-maker knows what he is doing.

 

[nzoomed]

lol

i know what you mean, yes this person has wound hundreds of trnasformers for audio amps over the years, so should not be a problem.

another thing im looking at is that i dont know what pfb is being used for, i expect its for presence control but correct me if im wrong, P level will be the main volume.

 

[flat5]

Pretty pictures:
**broken link removed**

 

[nzoomed]

yes i saw those pics earlier on, anyway does anyone have any advice for replacing the preamp stage with an ecc83 or 6sn7? im also interested to know if many people have used an ef86 in the preamp stage of a guitar amp before, and what you would reccommend, at the moment im coming up with several different concepts for the preamp and dont know what would be the best. TIA

 

[Nigel Goodwin]

yes i saw those pics earlier on, anyway does anyone have any advice for replacing the preamp stage with an ecc83 or 6sn7? im also interested to know if many people have used an ef86 in the preamp stage of a guitar amp before, and what you would reccommend, at the moment im coming up with several different concepts for the preamp and dont know what would be the best. TIA

EF86's have certainly been used in occasional guitar amps - but it's not common - you're looking for low performance, high distortion, poor frequency response, EF86's tend to be more the high quality amp market.

 

[nzoomed]

yeah i understand where you are coming from, i was thinking of using an ef86 for the clean channel and an ecc83 for the distortion channel, im still unsure if i should use ecc83 tubes or 6sn7

 

[Nigel Goodwin]

yeah i understand where you are coming from, i was thinking of using an ef86 for the clean channel and an ecc83 for the distortion channel, im still unsure if i should use ecc83 tubes or 6sn7

Can't help you on American numbers, but the ECC83 is a double triode commonly used for audio, in both HiFi and guitar amps. You can also simply plug-in the ECC81 or ECC82, which some people do for a slightly different sound, all are pin compatible (and I can still remember the pinouts from way back when )

 

[nzoomed]

lol valves are actually before my time, but i am sick of working on solid state amps, and after servicing the odd marshall and valve amps that come my way ive been so impressed with the sound that i want a valve amp of my own.
After doing a little reading, the 12au7 (ECC82) seems to be a later replacement of the 6sn7. so ill probably stick with ecc83, ive also found that bass amps prefer the ECC82 over the ECC83 as well.