First tube amp(6505+) question

[slothrop]

So I picked up a used 6505+ the other day which sounds pretty good but there's a constant noise and I'm hoping someone can tell me if it's just tubes or a bigger problem.

I'm not sure if it would be considered hiss or hum it's more like a constant shhhhhhhhh sound which happens when the volume is over 1 and the gain set to anything(even 0), happens when guitar is plugged in or not, the amp is plugged into a house outlet and I have tried several. I can hear it on the lead and rhythm channels but disappears when I turn off 'crunch' on the rhythm channel.

I also tried hooking up an hd500 using the 4 cable method and when I use amp sims there is no extra noise and everything sounds good. When I use a preset using the 6505's preamp I get the noise.

I'm going to bring it to an amp tech before returning it but any advice appreciated.

[RussB]

My son plays 5150's and has had several.

With any used amp, SOP is to replace all the tubes. In that series of amps, I like the Chinese 9th gen 12AX7's that are chosen for low noise (Mike @ KCA NOS tubes has them for a fair price) Pre-amp tubes are important in those amps due to their high noise floor, even with good tubes. Avoid the Tung_Sol 12AX7's, and they can & will fail in cathode follower positions. Position 1, 2 and 3 will have the most impact on tone, and subsequently, noise and hiss

I find them less sensitive to power tubes because the pre-amp generates all the crunch and distortion.

Here's the pre-amp tube layout,

5150 II/6505+

If you look at the rear of the amp, the preamp tubes are again on the right side.
Looking from right to left, the tubes are arranged as follows:
#1 (far right) the rhythm channel
#2 is shared by the rhythm and lead channel (input stage)
#3 and 4- lead channel
#5 effects loop driver
#6 phase splitter

[slothrop]

Thanks for the info Russ, I should have mentioned that this is a 1x12 combo with 5 preamp tubes and I guess the sound is more of a white noise.

Since I don't have any tubes lying around I brought it back to the store and they swapped all the tubes, the only one that made a difference was the 2nd from right(looking at the back) it helped a bit but there was still some noise.

They're sending it to their amp guy who I guess is one of the best in Mpls, they'll have him fix it at no charge to me which is fine since I got a great deal on this amp to begin with.