I think also that the DC-30 also needs some serious consideration for cooling. Mine has a heatsink and very small fan which thanks fully is almost inaudible. I believe its the same for most cathode biased amps. Fixed bias are eaiser to do aparently.

Yes, not quite sure where from, probably heater supply. You'll guess I didn't install PS myself

Started building today.

All is very straight forward.  The first few solderings were a bit shabby (out of practice) but it soon came back to me.

The quality of the kits is great for the money with just one exception - the power indicator (jewel lamp) has to be the cheapest and nastiest one I have ever seen.  I will be getting some good quality ones with a blue jewel for the amps.

We just did a lot of the fiddly stuff today, soldering the jumper wires and resistors to the tube sockets before installing them.  I have just twisted the wires for the heaters to leave them overnight, so they stay twisted.  Also paired up and twisted the PT output cables.   So, everything is ready to install the PT and lay the heater wires.

We picked up extra cable ties to keep things neater in the amps.

My friend the luthier is a degree qualified engineer but has never really done anything much with electrical.  It's great explaining to him how a tone circuit works and he just loves the physics behind the valves.  It's awesome being able to explain a diode, triode and pentode to someone who immediately understands it.  Our valves arrived from Bob at Eurotubes and my buddy has take the GZ34 home with him just to look at it.  I remember when I enjoyed just looking at valves.  I used to love cracking open the dead ones to see the guts of them.  They really are very cool devices.

Anyway, this exercise has put me back in touch with my teenage years.   It's great getting back into electronics (analogue style), I am remembering stuff that I assumed I must have forgotten over the years.  I think back to doing work experience at school with a TV repairer (yes, back when TVs were full of valves).

I'm loving it!

Cheers,

Crusty

Hi Alwalt,

Well I finally got around to finishing one of the amps.  We'll be double checking and firing up the second one next weekend.  Then it's on to building the cabinets.

I have an issue with this first one (don't know if it's a characteristic of the amp until I finish the 2nd) but there is a high pitched squeal at around 12kHz that kicks in at around 30% volume on both the 12AX7 and EF86 channels when the master volume is >50% or bypassed.

Although it's hard to tell at the frequency, it sounds like a sine wave and the frequency shifts slightly when the volume level is changed.

Given the nature of this squeal appearing within the preamp stage and the fact that it comes in when increasing the volume on either preamp channel when the master volume is up suggests to me that this is some HF feedback.

I'd appreciate any suggestions on how to track this down and eliminate it.

I already tried different EF86 valves from JJ EF806 to a few mesh plated Mullards that I have and it's still there, so it's doesn't appear to be EF86 related.

Cheers,

Crusty