EF86/TMB Mod Classic questions

[whboyd]

I've completed my build (but will post photos after cleanup LOL!). My voltages are high though. I used the Mojotone power section but with a Hammond PT, and substituted a 7H choke for the initial resistor (see supply pic attached). My line runs 123VAC and i've attached my voltages (also attached). The Robinette bias calculator shows me at about 10.3W dissipation per tube or 85.3%.

Before I finalize this I'd like remarks about the voltages, etc. Do I need to consider dropping the supply or does it look OK as is? Appreciate your thoughts and will post when completed.

[whboyd]

Another question regarding the EF86 circuit. Can you explain the reason for the two 470K resistors shown? They are in the (latest?) Modern Classic-ish schem, but not in the earlier EF86 Lite versions.
thanks!

[whboyd]

Not sure my earlier post of voltages uploaded properly so here's an image file of it.
thanks!
Bill

[Bieworm]

Search for the application of zener voltage dropping diodes. Get some 5 watt zenerdiodes in various voltages and daisychain them as shown on this thread: viewtopic.php?f=4&t=25724
This will take the B+ voltage down to where it needs to be.

The 2 470k resistors form a voltage divider network. It dumps voltage to ground to avoid too much signal reaching the next amplification stage. On some circuits this network is substituted by a 1M pot and acts as a gain knob. Check the Tremolo TMB schematic for this kinda thing.

[JMPGuitars]

Links in my signature.

Thanks,
Josh

[whboyd]

Thanks! I'll work on the zeners when they get here. Meanwhile a couple more quick questions... Most of the versions I see include one or two resistors right after the rectifier. Can you elaborate on these a bit? there is reference to a "sag" resistor but not universally used - then there's a 220K resistor to ground. Is this necessary if the voltages are corrected with zeners?

Also any suggestions on where in this scheme I would place my choke? It's rated at 70 mA.
thanks!~

[Daviedawg]

The sag resistor is meant to artificially mimic the effect of a valve rectifier in an amp with solid state rectification. The success is variable depending on the circuit and the listener. But your circuit has a valve rectifier so it is superfluous.
The 220k is a bleed resistor and in my opinion is a necessity. It bleeds the residual voltage in the big electrolytics to ground so you don't have the risk of zapping yourself if you forget to manually bleed them. After you switch off and go to do mods.

Dd

[whboyd]

Thanks! Anyone have a suggestion on where to place the choke (70ma)? I am going to reduce the B+ with zeners so would it be best to place the choke before or after the zeners, and further, should it go before the OPT CT or after (and just handle the voltages to screen, PI, and preamp stages)?

[JMPGuitars]

Thanks! I'll work on the zeners when they get here. Meanwhile a couple more quick questions... Most of the versions I see include one or two resistors right after the rectifier. Can you elaborate on these a bit? there is reference to a "sag" resistor but not universally used - then there's a 220K resistor to ground. Is this necessary if the voltages are corrected with zeners?

Also any suggestions on where in this scheme I would place my choke? It's rated at 70 mA.
thanks!~

SAG resistor is optional. It's to simulate a tube rectifier, if that's something you want. More sag is more bluesy, less sag is more rock.

220k resistor is not optional. That's to drain the filter caps when the amp is powered off to help you not die when you work on the amp.

I think the best location for a choke is in the dumpster. To each their own tho.

Thanks,
Josh

[yello]

I think the best location for a choke is in the dumpster.

Thanks,
Josh

Can you expound on this - why don't you like chokes?

[JMPGuitars]

Can you expound on this - why don't you like chokes?

What's the benefit of adding one? Anybody is welcome to experiment, but if there's no benefit, why bother?

More opinions here and elsewhere on this site: viewtopic.php?t=26217

[yello]

Can you expound on this - why don't you like chokes?

What's the benefit of adding one? Anybody is welcome to experiment, but if there's no benefit, why bother?

More opinions here and elsewhere on this site: viewtopic.php?t=26217

Thanks for the link, read that thread, interesting discussion.

So its mainly just no chokes in the 18w circuit or those that traditionally didn't have a choke, not a general no on chokes I gather? I don't know much about chokes technically, thus my interest to learn more.

[JMPGuitars]

So its mainly just no chokes in the 18w circuit or those that traditionally didn't have a choke, not a general no on chokes I gather? I don't know much about chokes technically, thus my interest to learn more.

Some circuits are designed with chokes on purpose. They probably make sense there. There are all kinds of electronic components that can be added to a circuit. But why? There has to be a purpose behind any change. If the purpose is to experiment, then make sure to document the circuit with and without the change, and then form an educated opinion on which way you like it better. NFBs and chokes have been experiement with on the 18W design, and the consensus was that the addition of those items resulted in a loss of the signature 18W tone. YMMV.

[Crabman]

Probably a historical reason for using chokes in the early years. However, for 100 watt amps running 4 power tubes, its probably makes sense to use a choke. The DC filtering is superior (less ripple) and if using a resistor instead you would have to be mindful of dissipation at peak load …. using a lower value of resistor would help with this but work against you at getting your noise rejection closer to 0 hertz in the RC filter. At least that’s the way I see it.

If you are into a lower power setup such as 18 watt then you don’t have as much current flowing through this filter node so it’s more feasible to put a resistor in that spot with large enough resistance to get reasonable noise rejection at that RC node without over dissipating that resistor at peak load.

Outside of that, you also have the feel factor. Never compared the 2 but it would appear that a choke will give a stiffer response than a resistor. The 18 watt vibe is all about squish, so if you combine a resistor in that spot with a tube rectifier you will probably get a much different response than if you used a choke with solid state rectifier.

I guess it comes down to what you like. The 18 watt vibe is all about the juicy chimey feel. A tube amp with SS rectification and choke will probably be better suited for immediate aggressive high gain tones.

[JMPGuitars]

Or an 18W with SS rec and sag resistor. But the sag resistor is to taste anyway. There's plenty of sag in the PT already.

[wmwelch55]

I have built several single ended amps and never was able to get all of the hum out 2 of my amps until I used a 9H 120 ma choke for the whole amp. It was dead quiet after that. I worked on the lead dress, I tried elevated heater, made sure I had no ground loops, I followed all of the suggestions about grounding. I know there were other things I tried, but the only thing that took away all of the 60 cycle hum was the choke.

After I fixed the hum on one of my amps I tried it on another one I was having the same problem with and had great success.

Push pull amps have some built-in hum canceling, but single ended does not, so I will not hesitate using a choke on single ended.

Mike