nasty buzz in old 35EH5 amp

[PRNDL]

I've been converting this very old Fanton amp to guitar and it now sounds great, except for the nasty buzz. It sounds like PS hum, but real loud.

I'm wondering if there's something off with the design, and have attached the schematics.

It has a simple diode rectifier, which originally was selenium. It also was directly connected to AC, but I added an isolation transformer.

I removed the feedback resistor to get more gain and distortion.

Any ideas??

 

[Eli Duttman]

Any ideas??

Yeah, that 1/2 wave rectifier places "standing" DC on the N-68X. Change to full wave bridge rectification using 4X UF4007 diodes.

BTW, where's the fuse protecting the N-68X's primary?

 

[Tom Bavis]

Are you using a star ground? There are fairly large 60 Hz currents flowing in the heater circuit and in the first filter capacitor. These should be returned directly to the transformer, NOT run through chassis ground or any other common ground bus. layout is VERY important in an amp like this... you have 120V to the heaters, which has to be kept away from the low-level stages..

Bypassing the cathodes of the voltage amps may reduce hum (from heater) while increasing gain.

Eli's points are also good - if you change to a full-wave bridge, you'll have to disconnect the transformer (and heaters) from ground, as the bridge (-) will now be connected to ground. And the heaters will have more voltage with respect to ground, so THAT source may be worse.

 

[PRNDL]

Are you using a star ground? There are fairly large 60 Hz currents flowing in the heater circuit and in the first filter capacitor. These should be returned directly to the transformer, NOT run through chassis ground or any other common ground bus. layout is VERY important in an amp like this... you have 120V to the heaters, which has to be kept away from the low-level stages..

Bypassing the cathodes of the voltage amps may reduce hum (from heater) while increasing gain.

Eli's points are also good - if you change to a full-wave bridge, you'll have to disconnect the transformer (and heaters) from ground, as the bridge (-) will now be connected to ground. And the heaters will have more voltage with respect to ground, so THAT source may be worse.

This is a very old amp (circa 1950 is my guess). It's a point-to-point nightmare! The grounds are tied together somewhat haphazardly.

 

[Eli Duttman]

Dude,

Change the on/off switch to DPST and switch both sides of the AC mains. Use FW bridge rectification, as the N-68X is most definitely not intended to tolerate "standing" DC.

Do not connect the series heater string to the N-68X's secondary. The heater string gets connected to the DPST power switch. To improve safety and eliminate the heat produced in a dropping resistor, use a series capacitor. Your tube complement uses 150 mA. and 95.2 V. That works out to an equivalent resistance of 634.7 Ohms. 120 V. working into an 800 Ohm impedance yields 150 mA.

Z² = R² + (X_C)²

X_C = 1/((2) (pi) (F) (C))

Work out the value of a 200+ VAC rated cap. to use in series with the tube heaters.

 

[PRNDL]

Thanks.

I get a little over 3µF using R=800-634.7=165.3

capacitance frequency resistance impedance Q
3.38876 µF 60 Hz 165.2 800 4.73824

http://www.cvs1.uklinux.net/cgi-bin/calculators/cap_imp.cgi

Put the 200V/3µF cap in series with the heaters.
Is this correct?

 

[Tom Bavis]

Almost... heater return is no longer to ground, but to the other side of primary AC. I'd wire it as: hot AC lead > cap > 35EH5 > 35EH5 > 12AX7 > input 12AX7 > AC neutral. Puts the least noise at the highest gain stage. I come up with 4.5 uF... I'd use a 4.7 uF 250VAC rated film cap. But then I have a few of those on hand... a 200 Ohm 10W resistor is really not THAT bad either...

 

[PRNDL]

I've rewired the amp with the bridge rectifier, although I haven't picked up the cap yet.

The buzz is still there. It changes significantly if I touch the transformer, which is physically mounted on the speaker cage.
Is it important to ground the transformer bolts?

At this point I'm wondering whether it might be better to scrap the old amp and put in an amp with 6V heaters. Maybe 6AK6's. Part of this decision is all the old parts and cramped point-to-point wiring.

 

[Tom Bavis]

Transformer case should be grounded, yes.

6AS5, 6CU5, 6BF5 should work in the same circuit without changes except for heater wiring.

 

[PRNDL]

6AS5, 6CU5, 6BF5 should work in the same circuit without changes except for heater wiring.

Based on your experience, which tube might be best for this amp, based on:
- tone (for guitar, with distortion)
- low power (1 watt per tube or less)
- availablility (NOS or new)
- price

I've tried a few and like the 6AK6 tone, although someone mentioned it is getting scarce. The 30C5 sounds nice but is a bit thin.

I've considered the ECC99. Although it is a bit expensive compared to others, it is new. I'm wondering how it will sound in a guitar amp. 5W is too loud.

Thanks!!

 

[Tom Bavis]

The 6AK6 isn''t hard to find in my experience... for two watts, it will do. 6AQ5 won't do much more at such a low supply voltage - needs 275V to get the usual 10-12W. I suggested the others since they're specified for 150V B+ - essentially they're a 50C5 / 50EH5 with 6V heater.

 

[Eli Duttman]

With the aid of a calculator, I'm going to compute the series cap. value for the Fanton.

Z = 800 Ohms Z² = 640000

R = 634.6666666667 Ohms R² = 402801.777777778

(X_C)² = 237198.222222222

X_C = 487.0300013574 Ohms

C = 5.4464455526e-6 Farads = 5.45 muF.

A 400 WVDC/5 muF. MPP GE 40L3501 should be OK in this 120 VAC/150 mA. regime. That part can be obtained from Madisound for 75 cents.

 

[PRNDL]

How about DC heaters?

I'm still having problems with hum, and am wondering if the 120V AC heaters are the main issue.

Is it possible to connect the heaters to one of the DC power supply nodes (the one closest to 120V with a resistor or cap, or 94V without one)?

The other option would be to get another transformer with 6VAC heater taps, and use different output tubes (as previously suggested).

I could also redesign the amp completely using readily available parts: A Champ 320V transformer, 6X4 rectifier, 6AK6 output tubes. I'm not sure if the output transformer would have to change as well.