MV-55 - most significant improvement yet

[RJKflyer]

My MV-55 sprang a hum, which was traced to a failed heater rectifier, or possibly a dry joint on one due to the heat.

I took the opportunity to do some replacements and upgrades:

1. Replaced heater rectifiers with Vishay SBYV28-100-E3/54 which are faster and softer recovery, plus lower dissipation - good given the heat damage to the PCB here.
2. Replaced the heater smoothing cap with a 10,000µ 105˚C Nichicon.
3. Added a WIMA 0µ1 across the DC heater rail between the two 6SN7s.
4. Replaced the two 4µ Solen PPE MKP with 4µ7 PPE MKP FC ('Fastcap'). Not sure this was necessary as the slight 'seepage' seen around the ends was in fact label adhesive. The caps were of zero leakage.
5. Added 0µ15 Solen MKP FC at the 'B' HT point across the existing 4µ7 - there are even holes on the PCB
6. Added 0µ15 Solen MKP FC across the 47µ at the 'A' HT point
7. replaced the 47µ 160V electrolytics with high grade Nichicons - one on the 'D' half-wave rectified supply rails and the other which couples the heater +ve to HT ground

The above were all low cost modifications/repairs/replacements.

Then I did the 'expensive' mods:
(a) Replaced the proprietary 0µ22 c-j PP caps in each phase signal line to the EL34 grids with 0µ22 Solen FEP (Tin/Teflon) - ca. GBP200 for the 4
(b) Replaced the 1µ proprietary c-j PP in each 6SN7 grid coupling to ground with 1µ V-Cap ODAM - ca. GBP120 for the 2

Result: one of the most significant upgrades I have done. It has opened more of a 3D vertical soundstage - which is what I normally experience as the glaring difference between RBCD and SACD - RBCD sounding albeit good but something of s 'letterbox' and SACD expanding the vertical image.

One of the most surprising outcomes was an otherwise disappointing RBCD becoming highly engaging with some audible details I had not previously heard.

Now, having made several changes at once, perhaps it's all down to the 50 cent electrolytics, or the two dollar bypass caps...

But I suspect not. It is a complete delight.

NOTE 1: Why did I go V-cap ODAM not Solen FEP for the 1µ - simply down to space available between holes AND between PCB and case top.

NOTE 2: incredibly annoyingly the outer foil indications on BOTH Solen FEP and V-Cap ODAMS were not as described by the seller, and RANDOM more to the point. So buyer beware. You don't want GBP200 grid caps in backwards!

[admin]

Thanks for the writeup. I may do an upgrade on my MV-55 in the future and there are definitely some great ideas here.

[Paolo]

My limited experience with an MV-55 was quite positive. I very much enjoyed that amp. Unfortunately it started to red plate output tubes and since I've grown quite weary dealing with vintage tube gear, I returned the amp for credit towards a Classic 62SE. While the 62 is clearly "better," I would not say that it provides me more musical enjoyment that the MV-55 did. But in "hi-fi" terms and parameters, I'd have to say that the 62 is the better component.