Marshall 2046 specialist weak reverb

[nilsampmaker]

Hi. I am a oldfart newbie (from Sweden) to this site. My hobby is to make old stuff work and now I have been restoring an old Marshall 2046. I have done a complete overhaul, made a turret board, and used new components except for the iron (PT, OPT and RT are the original ones). The reverb tank has earlier been changed to a long (old) accutronics type, so I am a bit unsure of whether itīs matched or not. Everything sounds real good but the reverb is a bit faint even on full tilt. I would deeply appreciate it if anyone could comment on this, the reverb drive is the pentode part of the ECL86 and recovery the triode. All component values are according to the schematic except I have used a little less filtering in the B+.

Cheers
Nils

[Robert M. Martinelli]

Hi Nils, and a heart-felt welcome to the forum!

The problem could be faced in more than one way; (other than trying to find an original reverb unit ). The first thing I'd try would be to add a cathode bypass capacitor to the recovery stage.

If this is not enough, be aware that the triode section of an ECL86 is similar to one of the two twin triodes found inside an ECC83, gm is 1.6 mA/V, mu is 100, this means that the internal Ra is also the same, 62500 Ohms. By taking some measurements ( the schematics that can be found on the Internet unfortunately don't show any voltages, e.g. the bias voltage across the cathode resistor and/or the HT ) you should be able to figure out the current operating conditions of that triode, and, by reducing the cathode bias resistor and/or increasing the anode resistor, increase the reverb recovery stage's gain ( which, judging by the current cathode resistor's value, 10KOhm, should now be very low ).

Hope this helps

Best regards

Bob

[nilsampmaker]

Hi Bob.

Thanks for the welcoming and your much appreciated help.

The original reverb tank was a short one mounted on the back of the chassis, and apparently not very usable, so I guess they often switched them out.

I will try your cap mod and get back with the result (probably after the weekend). I am guessing it will be enough because the reverb is just a tad to little today, for me anyway. So, thanks once again and Iīll keep you posted.

Btw, do you know if thereīs any information on old tanks to be had anywhere, or is it just trial and error when it comes to mounting position. Since itīs an old accutronics it doesnīt have the 9AB...... it only has four digits if I remember correctly

Best Regards
Nils

[Robert M. Martinelli]

Hi again Nils,
sorry, but I'm not keen on the Hammond model numbering system, OTOH I am sure someone else here surely is....
Cheers
Bob

[nilsampmaker]

Hi Bob.

I havenīt forgotten about you, itīs just that this is a hobby of mine so things take time. I want to thank you once again for your good advice.

The bypass cap mod worked out fine, personally I am very happy with the amplifier now. I will however get a friend to try it out and heīs very much into good sounding reverbs so I will get my verdict then.

I donīt know if itīs of any interest but I thought that Iīd publish the transformer codes.

PT, T3133
D.E
OPT, C2361
D.E
Rev, C2416
D.E

I wonder if the D.E stands for Drake or Dagnall, maybe you know?

Regards
Nils

[Robert M. Martinelli]

Hi Nils,
Glad to hear that the mod worked out, let's wait for your friend's "final" verdict...

As to the Xformer codes, those codes you posted seem to be Dagnall part number codes, but I'm not that keen on Marshalls, so I could be wrong, maybe someone more knowledgeable than me could chime in and confirm/deny this...

Cheers

Bob

[50bux]

hey nils. First... if its not broke dont fix it. Dont mess with a circuit that works great.My 2046 has great reverb and the circuit is bone stock.Tank is original mounted on bottom of cabinet.Its an Accutronics 4FB2A1C
17" 1475 Ohms impedance input and 2250 Ohms output impedance -medium delay and both jacks gounded. Standard old Fender style reverb is driven by two triodes tied together. Thats why its weak.
The old Traynors had an EL84 driven reverb and used the same basic model. They both are pentode driven and need a higher input impedance.