To much feedback with sv20-c

[Peveo]

it's been awhile but y didn" t forget the Roadhouse but y do need some help with an issue.
today y got myself a almost brand new sv20-c and although it sounds oke y do have some problems with it.
First...this thing is loud even in 5 watt mode , second it goes way to seen in distortion so y changed the ecc83 in v1 for 5751 to temper the gain a bit but.........if y use some overdrive pedals and modulation it's giving me a lot of feedback . and whistles..
My set up: sv20 -c is connected (8ohm) to a THD hotplate that's connected to 2x12 Marshall cab with V30's.
The feedback also occurs with another combo, a Harley Benton Tube 5( fun amp), if y add a pedal.
The funny thing is that y don't have any problem with my Jtm45 in the same configuration and the Jtm45 gives me a killer sound with it's Kt66's.

So...i'm a bit puzzeld here....is it my pedalboard, the THD Hotplate, cables that give me all that feedback with the combo?

Thanks,

[Cousin Muely]

I had the head. I never used an attenuator with it so I can't comment on that. It sounds like you may have too much gain going. I didn't use an overdrive with mine. Cranked it had plenty on its own for me. Just remember, it's not going to have the headroom of your JTM45. You might want to back the amp volume down some if using pedals to drive it.

[Dana,s blues]

With an attenuator like the hot plate, they can change the character of your amp. Get the amp to sound as good as you can without the hot plate. Probably best at 20 watts. Then use the hot plate to lower the volume. If you are attenuating in 5 watt mode you are probably getting too much gain. If you plug straight in to your amp do you get feedback? If not then something with your combination of gear in your signal chain is altering the frequencies of your amp. Good luck.
Dana's blues