Another theory on the 25-watt resistor.

[morcey2]

This is mostly just me thinking out loud, so feel free to ignore it.

I've seen several theories about why KF used a 25W 1k resistor where a 5W would have sufficed:

1. There is a significant AC component in the DC there. With an 80µ filter cap? Really?
2. There is some inductance in a wire-wound resistor of that size. Ok, I can buy that.
3. It's what KF had and it worked. (I've got about 250 820-ohm resistors that I try to use wherever I can. Instead of ordering 60, I ordered 60 5-packs of resistors. Maybe the same thing).

Here's my theory. Looking at the franchesca/kelly/other schematics, many of them have the 80µf Cap(s) before the standby switch. So when the power is turned on, they charge up nice and high, because there isn't any real load on the tranny. Maybe a couple of mA for the bleeders. When standby is switched to 'on', there is a massive rush of current from the 80µf cap to fill the other filter cap. I don't know what the actual dissipation over the time it takes to get the other filter caps charged, but the instantaneous dissipation as soon as the standby switch is flipped (assuming 450V B+) is 202.5 Watts! (450 * 450)/1000 = 202.5

That's assuming a purely resistive resistor (beats a purely resistive capacitor). Any inductance in the resistor will add to the 1k resistance and limit the current surge a little.

I've searched here and haven't found this theory presented anywhere. Any thoughts on this?

Matt

[lastwinj]

it won't be anywhere near 202 watts across that 1k resistor. maybe for the entire supply.

germ

[rooster]

Thoughts maybe, no theory.... First off I know you cannot find a choke with a resistive value of 1K ohms - something the circuit needed to reduce the screens to where he liked them, for $1. (The answer may be that simple, BTW.) Secondly, I am very sure he knew he would take some flak for using a simple resistor when everyone else who was charging $2K for an amp was using a choke. (Man, when you think about it, it must've been hard on the ego to be a tube amp Guru and go with a resistor here. Ya think?) So, if he was going to do it, he was going to be damn sure it actually worked very well, without a single failure. Peavey comes to mind when I think who else was doing this at the time, but they were not using 25 watt resistors, of course. And on that note, I have replaced a few in the large Peaveys, 5 watt resistors, actually, that didn't exactly fail per se, but that put out so much heat in the chassis inards area that they were dropping out of the pc boards - melting solder at the resistor leads, in other words. ..........So the larger 25 watt resistor will only get warm in the same situation, something that is also mighty important.

OK, that's all I got.

[geetarpicker]

I am always amazed at the dynamics of the Wreck. On my original and two clones if you really crank the amp hard and wack a chord you can actually hear the compression recoil after you stop playing. Even the hiss level actually jumps down then ramps up slow enough that you can actually hear it swell back after you stop hitting it hard. Almost like a compressor releasing quite slowly within say 200-1000ms. I think this is an effect of the power supply and contributes to the amp's ability to get such clear fairly dynamic cleans, and then get quite dirty tones that aren't much louder if barely. Like the amp really compresses hard at the output stage/power supply under full load, but opens back up power wise when you back off. Just my theory. I wonder if the resistor helps in this effect OVER a choke? The only other amps I've heard that did this were tweed fenders, like tweed bassmans with tube rectifiers however they tended to be much squishier and almost too much so when really cranked. It's almost like Ken was able to simulate tube rectifier sag without the tube. In some ways a Wreck "feels" like a JTM45 or tweed bassman, but a much more modern version.

[lastwinj]

I am always amazed at the dynamics of the Wreck. On my original and two clones if you really crank the amp hard and wack a chord you can actually hear the compression recoil after you stop playing. Even the hiss level actually jumps down then ramps up slow enough that you can actually hear it swell back after you stop hitting it hard. Almost like a compressor releasing quite slowly within say 200-1000ms. I think this is an effect of the power supply and contributes to the amp's ability to get such clear fairly dynamic cleans, and then get quite dirty tones that aren't much louder if barely. Like the amp really compresses hard at the output stage/power supply under full load, but opens back up power wise when you back off. Just my theory.

whats the current rating and regulation of the power transformer? that will have a good amount of effect on this.

I wonder if the resistor helps in this effect OVER a choke?

most assuredly. with a capacitor input filter supply, you have higher voltage at less current versus a choke, which you will get higher, more regulated current at lower voltage.


germ

[El_Martin]

Hi guys!

After a few hours of playing/tweaking TW style amps, I think Glen is right.
OK, Glen is years ahead as regards experience...and playing, of course.

Currently I tend to build TWs with "soft" power sections and/or not tuo much filterning. 40-50 uF for a two hole (2EXL84)pp Liverpool. Source resistance, filter capacity and load seem to be critical and have to be perfectly matched. I would consider most of the clones with oversized trannys and heavy filterin too stiff for my taste YMMV. With too low filtering the hum can be annoying OTH.
I remember a posting of Glen, where he said that his Express was not as loud as other TW clones of lower wattage. Is it in the older caps?

I have built Danas power mosfet thingy to tame the beast. I suspect my FETs resistance is too high. If I hit the strings hard w. engaged VVR, the amp sounds like strangled or choking. So I decided to put an additional single 22 uF F&T cap after the VVR to feed the whole amp. Away goes the choking

My Lil'Wreck (ECC83 + ECC82) has this breathing after loud guitar pulses also. You can hear the hiss coming back, while the caps are reloading.

Or maybe I'm completely off track

Ciao
Martin

[tubetek]

Let's remember a couple things about Ken's background...
He repaired TVs for a good while; no stranger to high voltages and reliability issues.
He worked for Ampeg for years; they known for gobs o' CLEAN power and pretty fair reliability (in that era, anyway)
When He went into business for himself (TWRECK) He (IMO) probably made a decision to over-design the unit for long-term reliability. Otherwise, why would He spec a PT that provided 300mA when 200mA would have "sufficed"?
Probably the same issue with the 25 watt resistor...When you are the only service and support point for your product, doesn't that make sense?
Also, the cost difference of $10.00 or so between using the larger PT and power resistor and a "spec" part is easily covered in a product that sold for as much as a 'Wreck" with as low an overhead as Ken had working at home, etc. It would pay for itself with less service on less units required.
Of course, I don't have firsthand knowledge of his real reasoning but I think all of the suppositions made in this thread have real merit.
(Damn, I LOVE stuff that makes me think!)
All the above is IMO, YMMV, etc.
tubetek

[morcey2]

Glen,
I think you're right about the resistor adding to the dynamics of this amp. I've got a Plexi (think JMP50) with 7591's that I built with a resistor in place of the choke and it's definitely more dynamic and touch sensitive than the other's that I've built with chokes, even though the chokes are pretty small (3-4H).



I just finished my first Express last night and we fired it up, but that's for a different thread. I'll just say that I was able to hear what you're talking about with the compression and was amazed to be hearing it in an amp with a SS rectifier and 94µF for the first filter cap. It sounds amazing. I did forget to hook up the center lug on the bright switch, so that didn't do a whole lot (duh).

Martin, as for Dana's VVR, I had something similar happen on my 18watt, but I only put in a 0.47µ cap before it and it kept the sag without sounding strangled. Dana is definitely the man on that one. Coolest amp thing I've seen in a long time. I've torn apart about 10 dead computer power supplies looking for suitable mosfets, but only finding regular power BJT's. And a stack of 800µF 200V caps that I've got to do something with.

Matt.

[jaysg]

He (IMO) probably made a decision to over-design the unit for long-term reliability. Otherwise, why would He spec a PT that provided 300mA when 200mA would have "sufficed"?

My notion about the PT is more that he didn't want sag. The output distortion is caused by running out of headroom, not a combination of the power supply's inability to deliver current and running out of headroom.

Take this with a pound of salt: back when KF was still making amps and Mike Zaite was posting on Ampage, Mr. Zaite quoted KF as saying that 'chokes choke the tone.' Something else to the effect that in KF's mind, 'what you don't put in your amp is as important as what you do put in.'

[jelle]

He (IMO) probably made a decision to over-design the unit for long-term reliability. Otherwise, why would He spec a PT that provided 300mA when 200mA would have "sufficed"?

My notion about the PT is more that he didn't want sag. The output distortion is caused by running out of headroom, not a combination of the power supply's inability to deliver current and running out of headroom.

Take this with a pound of salt: back when KF was still making amps and Mike Zaite was posting on Ampage, Mr. Zaite quoted KF as saying that 'chokes choke the tone.' Something else to the effect that in KF's mind, 'what you don't put in your amp is as important as what you do put in.'

I'm not a wreck expert but with that big PT I would guess that KF was aiming for well regulated powertubes' plates and a saggy effect in the rest of the amp due to the 1k resistor. This wil keep the tone better preserved at high levels than a tube rectifier that also sags the powertube plates. This means that the screens and nonals voltages are bouncing up and down.

Jelle

[jaysg]

I would guess that KF was aiming for well regulated powertubes' plates and a saggy effect in the rest of the amp due to the 1k resistor. This wil keep the tone better preserved at high levels than a tube rectifier that also sags the powertube plates. This means that the screens and nonals voltages are bouncing up and down.

Interesting thought. 1K is about 10x the resistive drop for a normal choke...to me the value is more suggestive than the power rating.

[dartanion]

Could the high wattage rating of that resistor also be associated with keeping the noise level down? As well, I would think that the larger resistor would dissipate heat better right?

[geetarpicker]

I've measured the B+ on my original Express both at the output tube plates and then upstream from there both playing lightly and cranked. The power supply isn't all that stiff on these actually, and the voltage at idle or light clean playing is quite a bit higher than when you really push the amp. I think the only way to have a truly stiff PS in a tube amp is with a regulated supply, which probably wouldn't sound so good for guitar. Interesting in the Express (or alot of other old tube amps for that matter) the whole power supply sags under load and even the bias voltage seem to modulate probably partially because it is tapped off the B+ winding too. There seems to be quite a bit of natural compression in the amp, and it's not just pure OT or tube saturation IMHO, but the limiting effect of the B+ ramping down when you have the amp cranked AND you turn the guitar all the way up and push the whole beast. Maybe the big 25 watt resistor adds to this effect, and the slightly heavier filtering makes up for it all in the ripple department. Maybe the resistor is there more for it's compression/sag effects on the B+, rather than to actually help with smoothing out the AC ripple. Pardon my ramblings as I have limited true mathmatical electronics background, just more of a guess on the theorys and a vague memory of talking to Ken about such things years ago. On a side note if you really crank the amp for long periods of time that 25 resistor can get quite hot actually. Probably a good idea to keep the wires running under it bent to stay down and not touching it. In my two clones I seached high and low to find the actual same brand (TRW) of 25 watt resistor as is in my original. I found some that weren't labled the same (IRC) but othewise had the exact look overall and wire lead size. Probably the same part from a different distributor. The original resistor in these has much thicker leads than the Xicons I've seen. Not sure if there are any other spec differences, but I tried to match the original exactly since you just don't really know what's inside them. Found some NOS ones on ebay a while back that appear dead on except for the inked on lettering.

[jelle]

I forgot to add: The idea is that the screen voltage drops more than the plate voltage. If the plate voltage sags more than the screens do, as can happen in an amp with a choke, then the electrons in the powertube get confused and do not travel to the plates but will get shunted to ground via the screens. This will suck soundwise, IMHO.

In an Express, I expect both plate and screen voltage to sag but the screens will sag more than the plates....I feel that Ken came up with a great responsive system

[ontariomaximus]

fwiw, I measured a drop of 40 volts at the node just after the 1k resistor when hitting the strings hard, and the volume at around 11 or noon. This was on one the Express clones I recently built, and had 6V6's in it.