Suggested hot cat / star amp mods?

[dubs]

Hi all,
I have a star amp gainstar 15 made by mark sampson.
I reverse engineered it and found the clean channel to be a lightning preamp and the gain channel to be a hotcat preamp. the power section is std el84 lightning fair.

The hotcat schematic floating arounds appears correct in the preamp section.
I've attached the gainstar values on the hotcat schemo for the dirty preamp. Ignore the power section since the 15 has el84 power section.

I'm finding the gain channel to be quite thick and to my ears muddy. Can't seem to get much clarity out of it. Any thoughts on mods? Change the tonestack? What effect will changing plate resistors have on tone (apart from gain)?

Thanks in advance

[booj]

try the different plate resistor idea. The 99 volts is a little low for guitar, but not for harp

[dubs]

It's been awhile but anyone have thoughts on this. I figured the tonestack was choking up the sound on the gain channel. I was right, I change the tonestack design slightly and this helped.

However, it appears the gain channel simply has too much gain and is very fizzy and muddy. Any ideas for further mods?

[JoeK]

Hi Dubs, I'm aiming at building a version of this amp myself. I'm sorry I can't offer any real advice about the problems your getting with the overdrive but I believe there are a few threads here discussing the preamp and the incorrect info on that schematic. The grid leak resistor 5M1 on the first stage is on the wrong side of the 68K grid resistor. The 1M and 5M1 should be reversed but your probably looking at the correct layout on your amp? Ceriatone have a layout for their "Straycat" but I haven't seen any clips posted on their forum yet. Stray Cat 30

[pdf64]

Components that are rolling off high end are the 1n5 and 470k in the V2b circuit, and the 360pF in the V2a circuit - consider reducing those values. However, too low and instability may result. The 1n5 is working as an 'anti bright' cap; as a 1st step I'd remove it and see how it sounded.
To reduce gain / fizz, consider reducing the 470k in the V2b grid circuit, the 150k in the V2a grid circuit, and the 270k in the V3b circuit.
Agree that the 5M1 and 1M in the V1a grid circuit should be switched around for adequate biasing. Pete.

[dubs]

Thanks Pete and Joe for your reply.
What about reducing the 220k anode resistor on v1a and v2b to 100k?
I gather the 470k grid resistor on v1B is to prevent any blocking distortion for a high gain circuit.

[SysCrusher]

I would give V1A a cathode resistor/bypass like marshall. 2.7k/.68uf. By pass the 470k after V2B with a 1n. replace the 100n on V2B cathode with 1uf. Maybe even change V2A cathode to a 4.7k - 10k for some marshall crunch. That should tighten it up for more clarity.

That's what I'd do but in the end, it's up to your taste.