Amp tinkerers and people who aren't afraid of 700 volts or so in here....

Moderators: Slowy, Capt. Black

User avatar
BY Optical
#807602
I just noticed the photo haha
The v1 cathodes arent linked that I can see, all pins look disconnected.

I thought the PI was all messed up then realised it's a cathodine circuit and another gain stage driving it.
User avatar
BY jeremyb
#807604
GrantB wrote:I made some toast this morning.


My toaster actually has more technology than this amp does :mental:
User avatar
BY AiRdAd
#807605
I think you guys should start making those changes on Slowy's amp, just to make sure you like the sound before you make yours! I'm sure Slowy won't mind :-)
User avatar
BY Slowy
#807610
AiRdAd wrote:I think you guys should start making those changes on Slowy's amp, just to make sure you like the sound before you make yours! I'm sure Slowy won't mind :-)

Danny...........I know where you live.
User avatar
BY GrantB
#807613
StrummersOfThunder wrote:
GrantB wrote:I made some toast this morning.

Yes I can hear you Clem Fandango !


YES, YEESSSS.

If anyone has watched Legion (Netflix - the mutant power one, not the other one) Oliver is super cool. Reminds me of Steven.
User avatar
BY RectifiedAmps
#807614
Thanks for drawing up the schematic!

I wonder if the drifted R7 value is what is giving this amp so much mojo? If R7 is measuring >1M then it would give a pretty big gain boost while rendering the tone controls more or less transparent, as Optical points out. Do the tone controls do much?

Also, the 1st half of V2 is a bit different - the 47k anode resistor would mean this stage wouldn't be swinging as much voltage as traditional stages (ie. lower gain), which may be by design so that the following cathodyne phase splitter doesn't get overloaded as much, which can produce nasty overtones.

I wonder if all this amounts to most (all?) of the distortion character being generated in the power amp, not the preamp? The use of a 12AY7 is a bit unconventional too - could this be a mod, with the original as a 12AX7? The 12AY7 would keep the preamp cleaner as well but it's not a typical preamp tube for any vintage kiwi amp as far as I've seen. 12AX7s, 12AT7s and 12AU7s seem to be the usual choices.
User avatar
BY Slowy
#807623
RectifiedAmps wrote:Thanks for drawing up the schematic!

I wonder if the drifted R7 value is what is giving this amp so much mojo? If R7 is measuring >1M then it would give a pretty big gain boost while rendering the tone controls more or less transparent, as Optical points out. Do the tone controls do much?

Also, the 1st half of V2 is a bit different - the 47k anode resistor would mean this stage wouldn't be swinging as much voltage as traditional stages (ie. lower gain), which may be by design so that the following cathodyne phase splitter doesn't get overloaded as much, which can produce nasty overtones.

I wonder if all this amounts to most (all?) of the distortion character being generated in the power amp, not the preamp? The use of a 12AY7 is a bit unconventional too - could this be a mod, with the original as a 12AX7? The 12AY7 would keep the preamp cleaner as well but it's not a typical preamp tube for any vintage kiwi amp as far as I've seen. 12AX7s, 12AT7s and 12AU7s seem to be the usual choices.

The 12AY7 is my modification. I prefer the sound. And you are correct; power amp distortion is what this does.
User avatar
BY GrantB
#807624
I have always wanted a Harvard!
User avatar
BY jvpp
#807632
Thanks Optical, SizzlingBadger and RectifiedAmps for your feedback.

R17 and R18 are indeed going to ground, just had not noticed the soldering pin was actually used as mounting pin to the chassis as well.
C5 - I have chosen a value in between what you suggested, so going with 120pF.
R1 - Yes, colour code says 1M, but I accidentally used the drifted value which is 1M6.
R7 - great spotting Optical - it is actually 6K9 on my hand drawn circuit but obviously when entering in Digi-Key I forgot to enter the correct value. And yes, the tone controls work fine.
V1 second triode - correct SB, is not biased.

SB - what do you mean with C6 being wrong? And what value would you recommend for the capacitor in the input circuit?

The updated circuit diagram will be posted later today.
User avatar
BY jvpp
#807633
Optical wrote:I just noticed the photo haha

I thought the PI was all messed up then realised it's a cathodine circuit and another gain stage driving it.


I also don't have a Harvard - so perhaps you would like to elaborate?

Long live the Valve Wizard :D
http://www.valvewizard.co.uk/cathodyne.html

So if I understand the Valve Wizard correctly, then R16 has a dual purpose: 1) creates the required cathode bias, 2) is the large gridstopper which avoids anode 'frequency doubling' in case the cathodyne is being overdriven. Yes? No?
Last edited by jvpp on Fri Nov 09, 2018 3:35 pm, edited 1 time in total.
User avatar
BY sizzlingbadger
#807634
C6 is fine, I too was thinking it was long tailed pair PI at first.

0.1uF should be ok on the input make sure R1 is on the tube (grid) side of the cap though so it can drain the DC.

Capture.JPG
User avatar
BY jvpp
#807635
Circuit Diagram Revision 1 courtesy of Optical, SizzlingBadger and RectifiedAmps.

What is the impact of R17 and R18 actually being 390K?

Image

Close up of C5 pipe capacitor:

Image