Bell Sound 2418 - bringing it back.... Advice?

[boeingpilot]

I was asked to take a look at a Bell Sound 2418 integrated amp to see what it would take to get it back into service.

A little background -- over the past 30 years I've taken care of the care and feeding of a Dynaco ST70 and PAS3X. Also brought 2 other ST70's and PAS's and a SCA35 back, but in each of those cases went straight to recapping the power supplies before I would even think of powering them up.

In this case, as the owner wanted to know if it was even close to working, I took the following steps

-- checked all tubes on my Jackson tester. All tested at 100% or better (geez, 4x west german telefunken 12AX7s!)

-- Physically inspected the unit for obvious damage (none)
-- Powered up slowly with Variac. Passed the no smoke test.
-- Introduced signal, and found that it worked (at 90V) the only noticeable noise was a dirty volume pot. Took care of that with DeOx. No noticeable 60 hz hum
-- Brought the unit up to 110V (which would've been US line voltage back in 1961 when this was new)

Sounds very nice connected to a pair of Heresy I's. However, started to get an issue... Found that when bringing volume above say between 1/2 and 3/4 range, I get rapid motorboating both channels. Problem is worse when increasing bass.

So -- I'm thinking the power supply is starting to breakdown under high load? I'm ok with dropping the $80 or so on new filter caps, but want to get a reality check on where else to look?

Ideas? Advice? Seems like it's a nice little amp (perfect for use with efficient speakers like the Klipschs)

 

[Tinkerbelle]

It sounds like you are on the right track. At the current age of the existing caps and assuming the intent is to continue use for several more years or decades ahead, replacement would be appropriate consideration. They could be contributing to the motor-boating effect. When you had the unit at 110VAC input, did you take any high voltage DC voltage readings for B+ etc.(350V, 320V, 240V, 170V sources to tube sections)?

If there is sufficient working volume underneath, it might be less costly/easier to place individual caps below the cans rather than replacing the multi-section can as a single piece and just bypassing the existing can so it is out of circuit.

Note: take usual safety precautions & actions if & when working with such high voltage.

 

[primosounds]

Yep, the PS can be borderline and replacing the original electrolytics would be the thing to do. JJ has a good price 2 and 4 section electrolytic that would work for most vintage amps if you want to go with chucking the old can caps. You can buy a clamp for installation and sometimes i will use epoxy glue to fix it to the chassis instead. You should also measure if there is DC leaking thru the coupling caps. If you want to be very thorough, check all the CC resistors that are dropping voltage or are in the PS. You will find that they have probably drifted over 10% from original value.

 

[boeingpilot]

Thank you - needed the reality check. Just ordered two multi-section cans (yea, I know I can throw discretes underneath, but this is for someone else, and I want it to look stock all the way through when I'm done.

Yea, the coupling caps have me nervous too. I guess I'll start with the filters and work from there. I was in a bit of a rush this morning, so didn't have time to check the voltage rails. I'll check that before and after the filter caps are replaced. If it keeps having issues (from my little theoretical knowledge) I'd say the coupling caps are the next place to go.

 

[boeingpilot]

Follow up (FYI) Replaced filter caps (couldn't get exact multi -cap - used closest for 4 out of 5 caps to retain original look, replaced one section under chassis with discrete cap). Solved all issues.

For a 15 wpc amp, not terribly bad. (would I call it a Dyna ST35, no, but not bad). Tested it on a pair of Heresy I's. Except for max volume, pretty good combination. Even a max volume could detect no audible distortion from amp.