Rectifier tube output loading (or not)

[pjd3]

Hello,

My next step in setting up my tube amp design board is getting the rectifier tube section all ready to go. My main concern is the implications of having or not having a load on pin 8, the rectifier output. Are things OK with not providing any load to the output of the rectifier tube? Or, is it necessary to provide some kind of load?

The power transformer I am using at this point does not have a 5 volt heater coil, but does have three 6.3 volt coils, two of them providing up to 4 amps of current and it is looking like I will be fine (according to ohms law!) with placing a .68 ohm 5-10 watt resistor in series with the rectifier tube filament (but I will keep the resistor away from the high voltage side of the rectifier filament just to be careful). I'd like to prove to myself that the .68 ohm resistor is going to eat up 1.3 volts of the 6.3 volts leaving 5 volts across the rectifier tube heater filament. I did create a post about this very topic and it seemed that most thought that this was a viable approach to heating the rect heater coil. Once I know that this is going to work, I can feel confident going forward with building the preamp and output sections.

Just to mention, I do have a particular amp project in mind but will also be approaching this "amp design board" thing as an educational tool to learn more deeply the math and technology behind tubes and their circuits. My aim is to see how different stages work and affect sound - different equalizing, number of gain stages, cathodyne verses long tailed pair PI, cathode followers before eq, all those different elements I've seen in the various topologies of the amps I've used and built. And hopefully, I will end up with an amp personally suited for one of my instruments that I built.

Thanks for stopping by!
Best,
Phil Donovan

[xtian]

The rectifier tube does not need to see a load. It's common to install only the rectifier tube in new builds to test B+ voltages before you install the rest of the tubes. Your 0.68R resistor is going to create a lot of heat, so plan accordingly (like, don't place it next to electrolytic caps).

You know also about the max recommended reservoir cap value following the rectifier? Which rect are you using, and what's your reservoir cap value?

[pjd3]

Thanks Xtian,

At this point it is a GZ34. The PT is 335-0-335 (200mA) so I believe I'm expecting a B+ of about 435 volts, maybe the EL34 plates will see around 420 volts or so.

The first types of circuits I will be building around are the old hi fi dynaco/mullard/williamson which appear to hit 40uF. (two 20uF in parallel). I'm looking at this schematic now and strangely I don't see any choke in the circuit. But then again, this is a hi fi circuit (EF86, 6SN7, EL34 pair), and I've only build 2 guitar amps. I'm just very curious to hear what this particular instrument sounds like through one of these circuits with alot of NFB and Ultra-linear OT. For all I know, I may not even like this sound but then again, thats why I'm doing this, to force myself into finding out what topologies work best for this application.

Thanks Xtian
Phil Donovan

[pdf64]

Have you checked on how much an OT for such an amp will cost?
It might not work out well if a simpler, cheaper one was swapped in; because there’s so much NFB, there might be little to no margin of stability.

And the amp design might not cope well with heavy overdrive, and might have poor ‘recovery from overdrive characteristics.

[pjd3]

Hey pdf64,

This is beginning with a hi fi amp and could very well venture far from that. I'm not even sure if the final result will be a common OT or an Ultra-linear. And there will not be any overdrive sought out at all. This amp is for an instrument I built which upon first glance makes you think its a guitar - and the form factor definitely is. However, it is a fretless that is to be used with an Ebow, only. No strumming, no picking, no overdrive, only clean and fairly non-distorted. I normally record this "guitar" through DI/Mic pre/reverb with A/D conversion into Reaper, and it's been working decent that way for recording. I plan to couple the instrument soon with IR processing, possibly a violin body resonance emulator to see how that works. Everything is experimental right now and I have no idea what status between hi fi and guitar amp that this will end up at. But, that's why I'm doing this. A unique amp for a unique instrument and hopefully learn a whole hell of alot during the process. I may even decide I need to move up to KT88's, that the EL34's aren't giving enough clean headroom. Plus, I really am curious how the different amp topologies respond to this "guitar".
I measured the output of the fretless Ebow guitar with a Fluke DMM and measured upwards of 3 volts for the highest note at full volume from the guitar. I probably should be determining that with an oscilloscope but, so far it looks like there is at least line level coming out the jack of this instrument. That's another reason I decided to make the old hi fi amps just a starting point.
Just on the OT, I see that the NFB for the hi fi amps is quite more involved than a standard guitar amp. I will be respecting the call for a multi-tap UL OT as the schematics call for. I like the looks of the Dynaco replacement OT's. (but not so much the cost). Thats OK, birthdays and Christmas always come around for more "wife battle leverage".

Phil Donovan