(Slow) Build log for Vox JMI-era AC15 OA-031 circuit

[pdf64]

...can I pull it off with just 6x 10M resistors, with one at each of the output end of the 6 capacitors like in the fourth diagram? http://cdn.shopify.com/s/files/1/017...ypass.png?1326

I figure the latter will work.

I guess I will order 12+1 10M resistors just to be sure I have enough on hand.

EDIT: Due to Sluckey's design, because the "to board" side of all the capacitors are tied together, a single 10M resistor could work on that side, so the total to order would be 6+1 10M resistors, rendering the above question mostly academic.

The current limiting resistors (1M in the article) pose a challenge. If I understand it correctly, I would need 1+6 1M resistors, meaning the rotary switch will be crawling with point-to-point-wired components*. But I probably don't want the current-limiting resistors anyway, since this is an amp and not a stomp box...

Here is a schematic of what I envision the "Sluckey's AC15/AC30 6-Position Brilliance Switch with Added Pull-Down Resistors" should look like:

[ATTACH=CONFIG]40816[/ATTACH]

Thoughts?

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Many apologies, I should have explained things better.
Yes, only need 6 resistors, to give the switched legs of the caps a dc reference.
As their other leg has the preceding plate as their dc reference; the issue being that with the switched leg waiting around to be connected, tiny dc leakage from the plate side will result in it getting charged up. Then when the switch is actuated and it's brought into circuit, that leg will discharge into the signal path.
Current limiting resistors are almost certainly not needed, plus they would likely affect circuit operation.

The slope control could be made more effective by using a higher value pot; 2M may be available, or use a dual gang 1M wired in series, even a 2M dual gang.
Or you could DIY the existing 1M track to a higher value with a bit of judicious scraping at the max resistance end of the track; similar to a 'no load' guitar tone pot, in which the track is completely broken.

[dchang0]

Thanks! I'm learning a lot here (never really paid attention to tone stacks before, because it was always easy to simply drop in the classic Fender tone stack without thinking about how any tone stack works).

I will continue building the "Sluckey+pull-down resistors" control AND try the 2M slope control. Whichever is better stays in the amp...

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Alpha apparently makes 2M linear pots, but I do not yet see 2M log pots. I don't trust Alpha anyway--had one pot fail when a leg came loose from the trace.

Here's a 3M reverse log pot that could work: https://www.tubesandmore.com/node/3837

[dchang0]

The slightly-microphonic Tung Sol gold pin EF806S has been tamed!

I can now crank the amp all the way to 10/10 and not get any of the high-frequency feedback. This particular tube will probably last over a year until its internals are shaken loose (the chassis vibrates a lot at full volume, but it is a low-frequency vibration).

Currently, the microphonic-taming setup is: two layers of heat shrink on the tube glass, one layer of heat shrink on the whole socket and tube, o-ring suspension mount. The plan is to remove the heat shrink tubing and replace it with either $5/pair silicone o-rings or the expensive $29/ea. Herbie's UltraSonic Guitar tube dampers for better air-cooling.

And so, all the problems with this amp have been solved.

The bass-shelving control has been reinstalled temporarily so I can use the amp; I will be ordering the parts soon for the mods pdf64 suggested.

[dchang0]

The CTS 3M reverse log pot from tubesandmore.com arrived. (Note to possible buyers: the bushing is a bit too short. I may have to abandon the star lock washer when I install the copper control panel.)

It made a noticeable difference in place of the 1M log pot in Merlin's bass-shelving cut control, as pdf64 intended.

I also received the Alpha 2-pole, 6-position MBB (shorting) switch. I'll build that out next. Based on my previous experience with the 6-position Brilliance coupling caps, I expect this to be the winner that stays in the amp.

First I will try sluckey's design (without the pull-down resistors) and then add the pull-down resistors to remove the pop.

[KevinOConnor]

Hi Guys

Regarding post-19:

The preferred fusing is to use two fuses, one for each end of the winding. Generally, every winding should have a fuse to allow true protection of the PT. Windings with more than two wires should have as many fuses as there are leads minus one.

The single fuse on the primary side only protects the mains NOT the PT and NOT the amp.

It is possible for both diodes to short in the rectifier scheme shown if one diode suffers an outright failure and the other is conducting due to leakge - called "cross-conduction". The latter is avoided if there is always a load on the rectifiers, made so simply by adding 330k to ground from the rectifier output. The current through this R will swamp any leakage currents and provide a path for them. It is not uncommon for the solid-state diodes to blow up when one switches to the tube if both types of rectifier are connected to the winding all the time and just their outputs are selected. The unloading of the SS diodes allows the cross-conduction to occur with one diode shorting and the other still viable if only briefly.

Make sure there is a bleeder across the caps and no standby switch, and this will prevent cross-conduction. if there must be a standby, add the R to ground from the SS diode output AND a bleeder across the caps. Add another diode to block the standby switch from the caps so the switch life is extended.

Have fun

[dchang0]

Thanks, Kevin-- What specifically do you mean by "bleeder across the caps?" (There is no standby switch in this particular build.) Is this bleeder across the caps installed in addition to the 330k to ground at the rectifier output or in place of?

It may easier to point me to schematics of amps that illustrate these safeties.

I also don't understand this: "The single fuse on the primary side only protects the mains NOT the PT and NOT the amp." I'm scratching my head trying to understand how the PT would not be protected by the single fuse as shown on the primary side. I'd appreciate it if you could elaborate...

[KevinOConnor]

Hi Guys

"bleeder" resistor - a resistor wired permanently across a filter capacitor that will automatically drain away charge when the power is turned 'off'. Every amplifer should have this in place.

dchang0 - If your amp has no standby, then the 330k added across the first filter cap will protect the diodes and remove charge at power-down.

A typical amp has a 3A mains fuse. How much current does the bias supply have to pull to blow that fuse on the other side of the PT? How about the power tubes? or the heaters? or any other auxiliary supply? A lot more damage results when there is only one fuse on the mains with nothing on any other PT winding. The fuse blows at 3A, so the mains is protected even though the inside of the amp is burnt to a crisp.

With individual and correct fusing for each winding, each is protected. The bias winding might be a 50mA winding, so it needs a small fuse to protect it if something in the circuit shorts. Without its own protection, if that something shorts then the winding will be irreparably damaged and the whole PT might be toast. My current Studio amp has six fuses: two for the dual primaries and the rest for secondaries.

Have fun

[dchang0]

Perfectly explained--thanks Kevin!

[pdf64]

Fuses seem to blow when they shouldn't, and not blow when they should
Hence fusing the bias supply may end up causing a problem, eg if the bias supply fuse blows for a spurious reason but the HT fuse/s then fail to blow quickly enough to the high current draw that would then result, to the detriment of the HT winding, ie it's not a 'fail safe' protection system.
So it may be seen that on balance it's best to leave the bias supply unfused.
But as the AC15 is cathode biased, this is a bit of a diversion!

[dchang0]

A question: if the 3A fuse on the primary PT winding is too high to protect the PT, will it be enough to simply lower its value and/or change it from Slo Blo to fast blow? If it is, then what value would make a good balance between being low enough to protect the PT and being high enough that it doesn't blow at inopportune times like in the middle of a solo?

I found a bleeder resistor calculator online and plugged in 330K and 16uF (C40), and it came up with 0.27W power rating (I have a Dale metal film 0.5W 330K on hand) and about 26 seconds to bring the voltage from 300V to 50V. That seems acceptable and very easy to install from C40's + terminal to the ground tab nearby.

[pdf64]

I think that what Kevin means by 'The single fuse on the primary side only protects the mains NOT the PT and NOT the amp' is that with our multisecondary PTs, as long as the other secondary windings aren't heavily loaded, any one winding could be passing a fault current without the PT's primary current being high enough to blow the primary fuse; hence good practice is to fuse each secondary.

Have a search for some threads with enlightening debate between RG and mooreamps on this topic, eg http://music-electronics-forum.com/t20404/

A PT primary fuse should be slow blow, as there will be an initial high transient surge current at power up, eg to set up the PT's magnetisation field; some amps seem to get away with quick blow fuses though.

Secondary loads tend also to have high initial surge current at power up (reservoir cap charge up, cold heater low resistance), and so slow blow fuses are the best choice.

[KevinOConnor]

Hi Guys

What I stated is what I meant and is the fact of the matter: The mains fuse protects the mains NOT the equipment with the fuse in it.

The mains fuse must carry ALL the current needed to support everything on the secondary side. Each secondary only takes a portion of that energy and should have its own appropriately sized fuse. IEC, CSA, UL et al require fuses on all windings although you see little if any fusing with SMPs. The regulatory agencies are not the law rather they are providing guidance at the best of times and are a nuisance at other times. There are still sensible ways to determine fuse sizes and positions.

ALL the fuses around the PT should be slow-blow as there are surge currents to accommodate even to magnetise the PT core let alone cap charging, heater warmup, etc.The fuse should be rated at 1.25 times the expected load current. This is because the current heats the fuse and "primes" it to blow prematurely if you rate it lower, so you avoid a lot of nuisance failures this way and still protect things.

The only place in a tube amp where you might use a fast blow fuse is for the OT itself - either side.

Have fun

[Justin Thomas]

I seem to recall Mr. Keen stating incessantly here and elsewhere that mains fuses are there to prevent HOUSE fires. There's no guarantee that your AMP might catch on fire, though... having had anamp catch fire without blowing the fuse, I see a bit of validity in this argument...

Basically, your fuse MIGHT protect your amp's trannies, etc., but if it does, it's just coincidence. And that is why we fuse all the secondaries!

Justin

[dchang0]

Makes sense. Thanks for the in-depth explanations. (I had forgotten about the reason for Slo Blo. They taught that in Amp Building 101, d'oh!)

[dchang0]

I finally wired up the sluckey 6-position, 2-pole MBB (shorting) rotary switch Brilliance coupling cap control and tried it out (previous testing was done with a BBM non-shorting switch).

First of all, yes, there is a loud pop between positions. The switch from BBM to MBB only solved the volume cut-out. I will absolutely have to add the six 10M pull-down resistors pdf64 described (those are pending an order).

Second, the capacitor values sluckey chose work well. There is a distinct difference between each of the six positions.

Third, sluckey's layout drawing of the Alpha rotary switch is backwards if you want Brilliance to "increase" as you turn the knob clockwise (most bass at the most counter-clockwise position). Flip sluckey's drawing left-right mirror image.

Fourth, I am not sure which control I like more, sluckey's 6-position or Merlin's continuous bass-shelving pot. Having the continuous range of adjustment with the pot is nice, but the specific taper of the pot matters a lot in terms of perceived difference between any position and another and the linearity of the knob. The discrete positions of the 6-position switch make the difference obvious and give a real sense of linearity.

I will probably keep the sluckey 6-position control since it is capable of reproducing the original AC15 Brilliance switch's choices. Plus, since I finally put in the effort to build the tedious assemblage, it is easier to leave it installed.



I'll post photos of the control with pull-down resistors when those eventually arrive.