Help come up with layout for this great sounding 5W amp?

[waldner]

Not able to give a contribute as to solve the fizzness or the oscillation but ....

what about to give a try to use the switch as to put ON or OFF C4 ? You can also use a center OFF SPDT and have two caps with different values at the place of C4

That's probably a better idea.  Simple and already in the right location for that switch.  Thanks!

[kagliostro]

OK for the SPST or SPDT on C4, may be also DPDT and act also on C3 ?

I'll give a try to the 4.7K pot in place of the switch (not that you must put it on the faceplate, only as to vary the cap insertion)

Franco

[SEL49]

I really don't know what's wrong with that cathode cap switch. It should work. I would need a sig gen and scope to troubleshoot further.

Meanwhile, I have a couple suggestions...

1. Move the preamp ground wire from the left end of the pot buss bar to the negative end of the preamp filter cap. Look at the pic. Then reevaluate.

2. Maybe forget the warm/bright switch and replace with this simple gain control. Works very well in my Smoky amp.

[waldner]

Ok, I spent most of the day tweaking this amp and I'm pronouncing it done- at least for now:)  First, a lot of the weird oscillation problem was related to the OT primary leads needing to be swapped.  It's weird because it was hard to tell through the small speaker at my work bench.  Nothing ever squealed or made it obvious.  The oscillation didn't appear till the volume was pegged all the way up.  Eventually I realized it was worth a try to swap the OT primaries just in case and from then on everything got way better.

I ended up ditching the warm/bright cap and used the switch instead for a choice of two cathode bypass caps, 25uf and .01uf.  This gives the amp two completely different flavors: cleaner and leaner vs louder and fatter.  I moved the NFB to the back panel and stuck with the 1K resistor.  This also gives a choice between more controlled and pretty ragged.  These along with the slope control, give this amp a pretty amazing array of choices and output levels.

It's the perfect home / studio amp.  The power is in exactly the right range and can go from whisper quiet to pretty darn loud with good tones at every level.  It can take pedals well but doesn't need them as it has great natural distortion above 5.  Highly recommended build.  Thanks to the mighty Steve for the wonderful schematic/layout and help, and to everyone else who participated too.  Very much appreciated!

I will have to record some clips to capture the various settings and sounds of the Mini 5. 

[kagliostro]

I ended up ditching the warm/bright cap and used the switch instead for a choice of two cathode bypass caps, 25uf and .01uf

So swapping the OT wires solved the oscillation problem and the change of the cap value is only due to your choice ?

Franco

[waldner]

Sorry I forgot two more things, I disconnected the NFB for awhile trying to troubleshoot the oscillation and ended up installing a snubber cap.  That actually solved the oscillation when the volume was pegged.   Then I went back and reinstalled the NFB.  Then changed the switch and started experimenting with cathode cap values.  I just kept alligator clipping in different values to experiment with the amp on and a guitar plugged in.  I ended up also changing the other cathode cap for V1 to a 1uF instead of a 25uF.  There was just more bass than necessary.


Not sure of the cause of the oscillation as I also tried moving wires, chopsticking, etc...  Maybe the lack of a metal chassis made it more susceptible to RF and interference?  The amp is as quiet as any of my normal amps though, and it was even when the oscillation was a problem, up until full volume.

[kagliostro]

A snubber ... but where

Franco

[waldner]

82pF black silver mica at the plate load resistors for V1.  I circled in red.

[kagliostro]

Thanks

Franco

[waldner]

Here is a demo of some of the sounds of this amp.  It starts out quiet so there is enough headroom for the louder settings to play without clipping the mic preamp.   Just my Nash Tele, a little bit of reverb from a pedal and a 1x12 with a Jensen speaker.  I left the slope control at about 75% the whole time.  Between the cathode cap switch, NFB switch, slope control, rectifier switch, and TMB tone stack, there are a lot of possibilities.  The distortion comes on after 4-6 on the volume depending upon all the other switch settings.

[waldner]

Ok, thought I was done but wanted to put a name plate on the amp.  After some thought I decided to put a leather tag with my logo (family crest) embossed onto some red Italian leather on top. 

Was also wondering some things as I have been playing this amp daily.  Let me know if you can shed light on either of the following:

- When switching between the diode & tube rectifier, the middle position of the switch is neither.  I have been using it almost as a standby, waiting a few seconds before making the change from one to the other.  What happens when you switch between them and there is a state of no rectifier?  Is going to the neutral position for a time before making the switch "easier" on the tubes or does it not matter? 

- Many of my other amps have grid stopper and/or screen resistors on power tubes.  Would either help with the 12BH7 or is it unnecessary?

Thanks!

[kagliostro]

https://www.valvewizard.co.uk/standby.html

Franco

[waldner]

Very interesting.  Here are some things that seem to be relevant:

"Valve (vacuum) rectifiers should always be allowed to charge the reservoir naturally from cold. If the valve is preheated before the reservoir is allowed to charge, the valve will have to supply the full inrush current when the switch is finally thrown. This is called hot switching and causes sudden cathode saturation that can lead to catastrophic arcing inside the tube. Hot switching of rectifier valves was usually forbidden by valve manufacturers. Opening a standby switch can also induce a ghastly flyback voltage across the transformer winding, large enough to cause arcing in a valve rectifier (a precaution against this is to add ordinary silicon diodes in series with each anode of the valve to reduce the reverse voltage across it). Yet more reasons why standby switches are bad news."


"Con: 1. Can lead to failure of the rectifier valve (if the switch is between the rectifier and reservoir capacitor);"


"If you have a valve rectifier then the standby switch must be placed after the reservoir capacitor so the cap can charge up slowly as the tube warms up.
Leaving the valves totally cut-off, while heated, encourages interface resistance. A simple way to discourage this is to add a resistor in parallel with the switch to allow a trickle current to flow at all times, while still keeping the amp more-or-less muted. A 47k 2W device is a reasonable compromise. You can also add a 100nF (or so) capacitor across the switch to reduce arcing inside it. It is hard to find (attractive) switches which are rated for high voltage use, especially DC voltage, so most people just use a suitably heavy-duty mains switch. Since the HT current is quite small (hundreds of milliamps, not amps), this does not seem to be a problem."

"When using silicon diodes we don't have to worry about hot-switching, so we can put the standby switch before the reservoir, which has certain advantages. Just as with fuses, switching a DC supply is much more stressful on a mechanical switch than switching an AC supply, because of arcing. Arcing leads to corrosion of the switch contacts and in extreme cases may even weld it shut. The switch will be subject to less arcing if is placed in the AC part of the circuit, e.g. prior to the rectifier. Here it only has to handle ripple current pulses, not continuous DC. Also, if a parallel resistor is added to allow trickle current then the diodes will enjoy a soft start as the reservoir slowly charges up through the resistor.'

So after consideration of these Valve Wizard ideas should I not ever "hot switch" between tube and diode rectifiers?  Instead turn the amp off and then switch.  Or is there a way, a la Vox's 47K 2W resistor, to make hot switching ok?  The last quote seems to indicate that hot switching from tube to diode is ok, but not the other way around?

[waldner]

With regard to grid stoppers on the 12BH7, I guess that's what the 1K resistors are?  I'm always confused as to whether resistors like these are needed in the circuitry or are acting as grid stoppers even though they are not mounted right at the tube sockets?  I read the Valve Wizard page on grid stoppers but much of it is over my head.  I also realized that the 12BH7 is a dual triode and there are no screens! Oops.

[kagliostro]

Add the resistor that you see on the Valve Wizard page, as you have a double rectifier you must use two resistors one for each brench


Franco

[SEL49]

My only concern about your STBY switch is the size of the switch. I would never use a mini toggle to switch B+. I would use this switch instead. Hey, it will even match your power switch. I suspect that after the newness wears off you will quit using that switch. Just decide which position you like better and leave it there. This little amp doesn't need a STBY switch. I'm curious, do you hear any difference between the diode and tube rectifier?

The 1Ks are the grid stoppers for the output triodes. You can move them to the socket but if you don't have any issues, I'd leave them on the board. I've built low gain amps both ways and never had a problem.

[waldner]

Good point about such a wimpy switch for B+.  Will go bigger.  The difference is subtle but noticeable.  The diodes sound slightly tighter but less harmonically rich and open.  They are close enough that I could live with either.  Thanks!