VCM's - Parasitic Oscillations - UK Vintage Radio Repair and Restoration Discussion Forum

David Simpson · 26th Dec 2013, 5:55 pm

This phenominon has been raised several times recently. I thought my LeyMarcAvo DC VCM was immune, having sucessfully tested & graphed Gm for several valves - up to 115mA on an EL37.
However, a recent aquisition of some TT22's were first put through my refurbished MK3, then into my L/M/A. The Mk3 handled them fine, but my L/M/A went hyper above 80mA on the best of the bunch. It produced a fine 29.28 MHz oscillation. Then shoved in another EL37, and lo & behold that produced a fundimental of 51.6MHz plus a healthy harmonic of 103.2MHz. However, raising the VBU on it's hinges an inch or so - stopped the par.osc.
Following some good advice from GlowingAnode - I managed to squeeze some ferrite beads underneath a IO valvebase in an extension box. Great result - no more oscillations even with the TT22 & the EL37 wound up to 100mA.
So from now on - all thumping big high-slope valves will be tested using the extension box, or another I'll build for 807's & QQV06's, etc. The B9G vb on my MK3 had single beads underneath. So in fact fitted 2 beads under each vb pin on the ext box & also secured them with some gash bits of h-shrink sleeving. I didn't fancy fannying about underneath the VBU on my L/M/A. Best to leave well alone if possible, I say.
Incidently, the L/M/A uses a modern bench DC PSU (PSM 3/2A) as an external heater supply - - christ - does it's display light up like a christmas tree when those VHF oscillations occured - obviously it's internal chips & processors etc - " dont like it up em" as Cpl Jones would say.

Regards, David

Jac · 27th Dec 2013, 5:36 pm

Indeed David, parasitic oscillations are just around the corner.

AVO used a large amount of ferrite beads in the testers.
Both in the valve-sockets area and near the selector switches.

It may help if you add a lot of them in your design in strategic places.

Jac

David Simpson · 27th Dec 2013, 6:46 pm

Right enough Jac, AVO were keen on ferrite beads, I've heard. My MK3 VCM does indeed have single ones/ pin skt on that big B9G v/b - above left of the most used IO v/b. But the donor MK2's valvebase unit which I fitted on top of my L/M/A didn't have any beads anywhere. The underside v/b to v/b wiring was so pristine I baulked at starting to prise apart the wires & un-solder the IO or any other v/b. Also,AVO's wiring size had a larger dia than the beads I have available, but thankfully they could be just about squeezed over the 0.5mm sq wiring I have in abundance. The beads came out of a Farnell packet marked 103002 as the part no. which I found in a gash box.
Incidently, ref to pictures 2 & 3, the co-ax scope lead which I connected across G & C sockets on the socket strip - stopped the EL37's P/O's when connected to my Hameg HM603, and even just a 1.2m length lying loose ! However, nothing stopped the TT22's until I fitted those two beads/ pin skt. That's a fearsome valve.

Regards, David

26th Dec 2013, 5:55 pm	#1
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,853	VCM's - Parasitic Oscillations This phenominon has been raised several times recently. I thought my LeyMarcAvo DC VCM was immune, having sucessfully tested & graphed Gm for several valves - up to 115mA on an EL37. However, a recent aquisition of some TT22's were first put through my refurbished MK3, then into my L/M/A. The Mk3 handled them fine, but my L/M/A went hyper above 80mA on the best of the bunch. It produced a fine 29.28 MHz oscillation. Then shoved in another EL37, and lo & behold that produced a fundimental of 51.6MHz plus a healthy harmonic of 103.2MHz. However, raising the VBU on it's hinges an inch or so - stopped the par.osc. Following some good advice from GlowingAnode - I managed to squeeze some ferrite beads underneath a IO valvebase in an extension box. Great result - no more oscillations even with the TT22 & the EL37 wound up to 100mA. So from now on - all thumping big high-slope valves will be tested using the extension box, or another I'll build for 807's & QQV06's, etc. The B9G vb on my MK3 had single beads underneath. So in fact fitted 2 beads under each vb pin on the ext box & also secured them with some gash bits of h-shrink sleeving. I didn't fancy fannying about underneath the VBU on my L/M/A. Best to leave well alone if possible, I say. Incidently, the L/M/A uses a modern bench DC PSU (PSM 3/2A) as an external heater supply - - christ - does it's display light up like a christmas tree when those VHF oscillations occured - obviously it's internal chips & processors etc - " dont like it up em" as Cpl Jones would say. Regards, David Attached Thumbnails

27th Dec 2013, 5:36 pm	#2
Jac Heptode Join Date: Jun 2006 Location: Eindhoven, Netherlands. Posts: 642	Re: VCM's - Parasitic Oscillations Indeed David, parasitic oscillations are just around the corner. AVO used a large amount of ferrite beads in the testers. Both in the valve-sockets area and near the selector switches. It may help if you add a lot of them in your design in strategic places. Jac Attached Thumbnails

27th Dec 2013, 6:46 pm	#3
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,853	Re: VCM's - Parasitic Oscillations Right enough Jac, AVO were keen on ferrite beads, I've heard. My MK3 VCM does indeed have single ones/ pin skt on that big B9G v/b - above left of the most used IO v/b. But the donor MK2's valvebase unit which I fitted on top of my L/M/A didn't have any beads anywhere. The underside v/b to v/b wiring was so pristine I baulked at starting to prise apart the wires & un-solder the IO or any other v/b. Also,AVO's wiring size had a larger dia than the beads I have available, but thankfully they could be just about squeezed over the 0.5mm sq wiring I have in abundance. The beads came out of a Farnell packet marked 103002 as the part no. which I found in a gash box. Incidently, ref to pictures 2 & 3, the co-ax scope lead which I connected across G & C sockets on the socket strip - stopped the EL37's P/O's when connected to my Hameg HM603, and even just a 1.2m length lying loose ! However, nothing stopped the TT22's until I fitted those two beads/ pin skt. That's a fearsome valve. Regards, David