Hoffman Amplifiers Tube Amplifier Forum

Other Stuff => Solid State => Topic started by: bphilxyz on August 16, 2022, 02:53:11 pm

Title: Crate GT-80 DSP - Switching from SS to Tube automatically
Post by: bphilxyz on August 16, 2022, 02:53:11 pm

Hello all,
   Hopefully someone can help.  My son's Amp is switching from SS to tube automatically.  Any ideas why?  And if so, how can I fix this issue. :help:

Title: Re: Crate GT-80 DSP - Switching from SS to Tube automatically
Post by: shooter on August 16, 2022, 04:20:33 pm

typically we like a schematic, comes in handier than guessing


is it relay switched, or mechanic switched, or both ?


a "scattering" relay could be crappy voltage, bad blocking diode.
bad connectors or solder joints can also be good guess

Title: Re: Crate GT-80 DSP - Switching from SS to Tube automatically
Post by: PRR on August 16, 2022, 05:38:40 pm

For anybody who does not have GT-80 details memorized:

There may be more than one version, this is tagged 'original':
crate_gt-80_dsp_original_sch.pdf
In general it is hard to tell what paper goes with what name. Here's a user manual:
https://www.manualslib.com/manual/33088/Crate-Quadradrive-Gt-80dsp.html
Another manual?
https://preview.manual.guru/preview-manual-for-free-359041/not-specified-crate-amplifiers-gt-80dsp/page-1.html

I'm not convinced these are the right docs, or error-free. The schematic is badly scanned and stitched. However the JFETs near TP25 seem to do the SS/Tube switching. My hunch is that a short on Q8 D-S will jam it in Tube mode. Replacing Q8 and C14 may be a next step. Wish I knew where (3) went. Examine D52 D53-- in a Fender "DSP" these diodes cook their joints and fail in a few years.