Cathode Tuning.

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Cathode Tuning.

Post by revolver1 » Fri Oct 01, 2021 2:29 pm

Hi all, been doing a little more experimenting and thought I'd share my findings. This will probably be more of interest to those who have just built or are thinking of building an amp but are relatively new to it all.

I was having trouble getting the sound I was after out of my 69 1987, I've read of a few other builders suffering similar problems. It was sounding quite harsh and pedals were sounding a bit crap and really not interacting well with the amp. I ended up pulling the .0022uf coupling cap and installed a .0047uf to let through a little more bottom end and I also put a 12at7 in v3 for a touch more head room and had 47k negative feed back on the 8ohm tap. It actually sounded pretty good like this, a little unique but it started to bug me a bit because I generally like to build things close to what may have left the factory.

I put the NFB back on the 16ohm tap to replicate the use of one cab with the NFB connected to the speaker Jack or output tag of the Ohm selector as the majority of units I've seen on amp archives have. I re-installed the .0022uf cap and put the ecc83 back in for a standard base line for the test.

One thing I noticed with the 820ohm Piher and Iskra's I had was the Piher ranged from 807ohm to 822ohm and the Iskra 819ohm to 846ohm. The carbon comps I have read from 830ohm to 900ohm but I wanted to avoid using those, I find the Cc's tend to loose punch and clarity.

I read as many threads on the subject of cathode resistor values to try make some sense of it and there was a great one on here about measured values in a real 68, 12000 series. The information seemed to point to a pattern that V1a seemed to be biased hotter than the cathode folower on V2a.

This kinda made sense to my to my non technical way of doing things, I'm just a DIYer with no technical back ground so I just experiment learn, ask questions and read alot around here.

From what I understand the first stage is where most of the clipping happens and not so much on the second stage cathode folower so I figure bias V1 for gain and cool the bias slightly on V2 for more head room in the second stage signal re-amplification.

With this in mind originally I had 820ohm on V1a and 822ohm on V2a. So I pulled these and put a 831ohm on V1a and a 1k on V2a, both Iskra.

For me the difference was a huge improvement. The amp sounds balanced and just all around right to my ear. Pedals blend perfectly, effortlessly and every thing just falls into place. The bottom end is much improved but with the cathode set up this way I'm not looking for the spongyness of the 8ohm tap.

Settings were channels jumped,

Pres 5
Bass 4
Mid just before 6
Treble 6
V1 4
V2 6

So then with this revelation I decided to revisit my 69 Superlead and late 67.

On the Superlead I had a 830ohm on V1a and a 820ohm on V2a, so I pulled the V2a 820ohm and put in a 841ohm both Iskra.

Again a huge improvement. Alot of people said you probably wouldn't hear much difference between a 820ohm and a 1k but to the contrary I found a very noticeable difference even changing V2a from 820ohm to 841ohm. I guess its subjective but for me it's the icing on the cake.

Now to the late 67. This was a bit trickier. Initially it was just too gained up and didn't sound at all vintage to my mind. I messed around with this one quite a bit. I went from the high voltage P.T taps to the lower voltage. I tried 32uf on the mains filtering and really liked what it did except it ghosted too much to play infront of anyone so that went back to 50uf but it was slightly stiff.

I had a 10k on the preamp dropper, I was hoping this would compensate some what for my preamp plates that came in at 98.8ohm. I ended up pulling the 10k and put in a 2w 8.2k piher that came in at 8.04ohm so a slight extra bit of voltage hitting the preamp and plates but no big deal but it still didn't sound right or work very well with fuzz and pedals in general.

V1a was 819ohm and V2a was 821ohm so I pulled these and put a 830ohm on V1a and a 836ohm on V2a. This was a step in the right direction but it was a bit stiff and a little bit of a struggle to play. I also had to nudge the gain a bit over 7 to get it going so I decided it needed a touch more gain to get it right.

I changed V1a out for a 821ohm Iskra and left the V2a with the 836ohm . Wow!!! What a difference this is exactly what I was after. It sounds gritty and really vintage not even a struggle to solo with a nudge of the guitar volume knob.

Rolling the guitar volume back I was getting a bit of Trex, Get It On and Bowie, Gene Genie sounded great. I then shifted to a bit of Cream, Free and Humble pie and it was really nailing it. Pedals work perfectly especially the fuzz. The whole amp sounds just all around better balanced and the feel is great, the drive has a broader kinda grit to it very appealing vintage tone crisp on top with plenty of throb. It even does a pretty convincing Malcom Young, what a great improvement. ZZ Top, Waiting For the Bus, La Grainge and Tush were hitting the mark and with my Schaffer Replica it was doing a pretty convincing Joe Perry. When dimed it all sounded great, I don't usually play it that way but it's nice to know it will do it and sound awesome.

Settings, top left treble boosted input;

Pres 6
Bass 2.5 to 3
Mid 10
Treble 7
V1 7
V2 0

In conclusion I think there is definitely something to be gained from tuning the cathodes and especially cooling off V2a in relation to V1a.

Hope this may be of some use, Happy tuning. :thumbsup:

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Re: Cathode Tuning.

Post by revolver1 » Fri Oct 01, 2021 3:02 pm

Forgot to mention in regard to my observations of the Piher resistors reading closer to 820ohm than the Iskra.
I know there are other factors that make the 70's amps more aggressive than the 60's but I can't help thinking the Piher's play a big part essentially biasing V1a hotter. Just a thought.

Also the fact my 67 seems to work well with a hotter V1a just that combination with that circuit gives a lift that stops it being stiff.

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Re: Cathode Tuning.

Post by Freebird » Sun Oct 03, 2021 12:37 pm

Thanks a lot for sharing your experience. I didn't think that such small difference in cathode resistance can make such difference in sound.


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Re: Cathode Tuning.

Post by neikeel » Tue Nov 09, 2021 4:21 pm

Very interesting
What did you do with V1b cathodes, and did that make a difference?

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