A unknown change done to the Leak ST50

[6B4G]

Hi guys
(rebuild)The amp parts were kept to all original values as I'm keeping it as original as possible (just as a sentimental amp)
I haven't finished with its preamp (Varislope Stereo) yet, just waiting for some electrolytics.
Anyhow I ran the amp with another tube preamp I built a while back and it sounds great, there was
a small issues that I had, I connected the preamp to the amp with a set of everyday RCA cables and I was getting
lots of noise coming out of the speakers, I then changed them with a better quality set and that dropped the noise by 80% (but still have an issue).
The amp had a mod done, when? I don't know, but they looks old at least 30 or so years old. Anyhow the mod is the addition of
a 250k stereo pot from the RCA inputs with a 220k resistor on the wipe to pins 2 and 7 of V1 the 12ax7 and also the R2L/R
has been changed to a 220k resistor, the schematics show a 22k resistor.
Can anyone shed some light on this?

cheers

[Geek]

Take a look if anyone put a 10R resistor or something in series with the signal ground? I've seen that in some Pass based designs as well as the old original schematic ST-70 causing the problems you describe.

[6B4G]

There's a 1Meg resistor to ground that is connected to the octal input for the preamp which is also connected
to the RCA input.

[6B4G]

The way I thought it should be is removal of the 1Meg and return the 220k to a 22k
But if the old preamp is to be used then ?????

[johnf]

The 250k pot is normally what you find on a TL50+. That is a different circuit with an EF86 as the first tube.
Maybe try putting the amp back to original with 1 Meg to ground and the 22k to the ECC83 then lose the pot. I assume they must have drilled a hole for the pot? The 2 methods of input (RCA socket or umbilical from Varislope preamp) should be in parallel, with either seeing the same 1 Meg load. Is it possible that the 220k was a mistake? or somebody trying to lower the gain?
Leak amps seem to have a single ground point on the chassis that is both circuit and safety ground -it is right by the input socket. So might be worth checking that they didn't add an extra one or do what Geek suggested.
There are some mods out there to lower the input sensitivity (and noise). People often do this to ST20 etc as all the amps had 100-150mV sensitivity. The ST60 had a hi-lo sensitivity switch added late in its production run to do that.
Obvious I know, but the octal socket for the umbilical is carrying the HT and heaters to the preamp as well as the screened signal cable so you might want to cover it if you don't use it.

[6B4G]

Thanks John, as I did a rebuild not long ago to a 50+ with the help of others I had also added a 250k pot and it worked great
re this amp it would be correct to return the amp back to original specs just to hear it and then I might add the mods to
hear the difference.

[johnf]

Have a look at Keith Snook's Leak pages. He discusses the input sensitivity issue in some detail. Including something as simple as removing the cathode bypass cap from the first stage.
I'm sure someone else here can explain in detail why just trying to lower sensitivity with a potentiometer has effects on the frequency response (I think?)

The TL50+ was designed with a 250k pot at the input. These were broadcast or PA amps. I think they often lived in the bottom of a speaker cab. The Leak blurb boasted about how many they sold to various broadcasters.
Good luck with the rebuild.

[6B4G]

These were broadcast or PA amps. I think they often lived in the bottom of a speaker cab. The Leak blurb boasted about how many they sold to various broadcasters.

The last two TL50's I worked on did in fact come from a local TV station.
You would think that these amps would be great, but till now all the ones that I have used and heard left me with questions.
They all seem to have gain issues that have to be looked at.
Thanks again for the help.