5150 Mod Thread

[TheWinterSnow]

Undersized screen-stoppers aren't good for stability either.

Especially if they're metal film, since below about 470R they'll typically show an inductive peak at RF. 1k and up should show a purely capacitive rollof in RF impedance. Carbon comps (which I assume Peavey used) don't look inductive at any value, but they're noisy.

Anyway, you could go much higher than 1k before you'd be dropping too much voltage. The tonal change due to increased dynamic compression would be more likely to be a problem before then.

Peavey uses 100R5W ceramic house wirewound resistors for the screen grids, so they are inductive. I found it odd enough that they deliberately used 5 watt resistors as to discourage bias modding the amp, as at normal biasing you are pushing 20W into a 5W resistor and still over 5W when the amps hasn't been biased, which explains why they blow when tubes go bad, regardless of bias. The remedy of that obviously is to reduce current on the screen so those resistors aren't running as hot. That would also benefit that fact that you would be dropping the screen voltage so that it doesn't match the plate voltage, which is a plus the way I see it.

Yeah. You could always place the choke in addition to the resistor, though. Or go crazy and use a BJT/MOSFET regulator.

My idea is to add the choke in series to the resistor, increase the SGR to 1K and then play around with the value of R210 (the resistor in series with the choke) to get the desired screen voltage and hope that it doens't mess with the preamp voltages too much, which it really shouldn't as if anything I would be reducing the value of R210 if anything)

Does kinda suggest they were getting a bit hot.

I don't know what the owner before was doing to the amp, but he replaced all the preamp tubes with GT 12AT7's, two of the SGR's where not working which meant only two of the tubes where working. And the power tubes didn't match. Three of the tubes are 6L6WTX+, two being Sovteks and one being a Generic Russian WTX, the fourth was a Sovtek 6L6WTX. I think the dude blew out the tubes, would buy one at a time (instead of a matched pair) and then sold it when he realized that two of the tubes weren't working. Fortunately it was only about $60 to fix the SGRs and replace the tubes with JJ ECC83s including a balanced tube for the P.I. Bringing tyhe total of the amp to $700, which I was happy with none the less. I have found that one of the power tubes has a crack in it although it is not yet leaking, soon enough though all four of them will need to be replaced, which I am surprised they lasted this long as I have been running them at 70% for almost 2 years.

 

[Omega_Void]

Adding a higher resistance on the screen should decrease the screen voltage therefore increase grid-plate capacitance, which at least to my knowledge is a bad thing as is lowers the amplification of the tube.

Actually, the grid-to-plate capacitance shouldn't be increased by dropping the screen voltage anyway. Provided the screen voltage is between the grid voltage and the plate voltage, it will still act as an electrostatic shield between them.

 

[TheWinterSnow]

Actually, the grid-to-plate capacitance shouldn't be increased by dropping the screen voltage anyway. Provided the screen voltage is between the grid voltage and the plate voltage, it will still act as an electrostatic shield between them.

I just read the section on radiotron. While having a screen voltage greater than grid but less than plate eliminates grid-plate conductance. Plate current depends on screen voltage and only to a slight extent of plate voltage. To paraphrase, higher screen voltages allow higher plate current.

It doesn't really matter how low the screen voltage is as long as you can get the amount of plate current that you want when biasing. But again you don't want to go too low as that reduces maximum plate current.

 

[sprack]

Peavey uses 100R5W ceramic house wirewound resistors for the screen grids, so they are inductive. I found it odd enough that they deliberately used 5 watt resistors as to discourage bias modding the amp, as at normal biasing you are pushing 20W into a 5W resistor and still over 5W when the amps hasn't been biased, which explains why they blow when tubes go bad, regardless of bias.

I put 2 200R5W in parallel in place of the wirewound ceramics.

 

[TheWinterSnow]

^Damn that is pretty creative, why didn't I ever thing of that? How did it change the tone if any at all. I imagine that it had to of changed the tone a bit considering that you are eliminating the inductance to the screens. Are You running stock bias or has it been modded? And WTF is that burned out node between the two far right tubes did you burn something out?

 

[Omega_Void]

It doesn't really matter how low the screen voltage is as long as you can get the amount of plate current that you want when biasing. But again you don't want to go too low as that reduces maximum plate current.

Yup. Maximum power is dictated by the screen, not the plate.

Screen current will only be about 25mA max or so. Drops 25 volts over a 1k screen-stopper, out of about what, 500 volts? Not a big deal IMO, 5150 has headroom to burn anyway.

 

[TheWinterSnow]

Yup. Maximum power is dictated by the screen, not the plate.

Screen current will only be about 25mA max or so. Drops 25 volts over a 1k screen-stopper, out of about what, 500 volts? Not a big deal IMO, 5150 has headroom to burn anyway.

475v @ screen seems realistic in terms of other amps I have seen including Marshalls and Engls when plate voltage is 500v.

You still have a tone of play on the negative control grid bias anyway to keep your idle dissipation up there, but still you run into the situation where you hit saturation earlier, which depending on the player maybe a good or bad thing. I personally don't know how to feel about it myself. I love poweramp saturation but too much sounds absolutely disgusting.

 

[Omega_Void]

Peavey... used 5 watt resistors as to discourage bias modding the amp, as at normal biasing you are pushing 20W into a 5W resistor and still over 5W when the amps hasn't been biased

Um, no. Pulling 25mA at max and probably abput 5mA at quiescent, the 100R screen resistors will be generating 0.065 Watts of heat at most.

Even allowing that they'll have to dissipate extra heat coming from the power tubes, 5W is a fair bit overrated I'd say -- or it would be if 100R was big enough to prevent the screen voltage exceeding the plate voltage.

Since it isn't the screen current will start running away when the amp is cranked and dissipation goes through the roof in the resistors and in the screens themselves, hence the need for silly-big power rating, but the screens still suffer.

475v @ screen seems realistic in terms of other amps I have seem including Marshalls and Engls when plate voltage is 500v.

If you want to work out the exact "optimum" value you need to measure the DC resistance of the output transformer primary from plate to centre-tap. Screen resistor should be as many times larger than this as the screen current is smaller than the plate current.

Definitely ditch the wirewounds though. Nice one, sprack.

 

[Omega_Void]

I personally don't know how to feel about it myself. I love poweramp saturation but too much sounds absolutely disgusting.

Yeah, but increasing headroom means biasing colder, and crossover distortion is worse than overdrive in my book.

As long as you still have enough power on tap to give the speakers a good kick up the arse, I say bias as warm as you can.

But it's personal taste, you got to find the balance that suits you.

 

[TheWinterSnow]

Yeah, but increasing headroom means biasing colder, and crossover distortion is worse than overdrive in my book.

As long as you still have enough power on tap to give the speakers a good kick up the arse, I say bias as warm as you can.

But it's personal taste, you got to find the balance that suits you.

As soon as I did the bias mod to mine I cranked the bias to 42mA and I liked it that way. I know a lot of guys in the forum don't like going over 32mA, but personally, I like to stay clear the hell away from crossover, and 42mA seems to work great. I just hope beefing up the SGRs is not too much to kill the usable headroom and thus volume. I am hitting saturation at about 3.5 which is typical rehearsal volume, I really wouldn't want to get into saturation sooner.

 

[Omega_Void]

...I just hope beefing up the SGRs is not too much to kill the usable headroom and thus volume. I am hitting saturation at about 3.5 which is typical rehearsal volume, I really wouldn't want to get into saturation sooner.

Off the cuff I reckon you'll lose less than a dB of headroom going up to 1k SGRs, 'cos you're only dropping about 5% off the voltage at most.

You should ditch the wirewounds anyway, so I advocate you try 1k first and if it doesn't work for you, go smaller.

 

[TheWinterSnow]

Off the cuff I reckon you'll lose less than a dB of headroom going up to 1k SGRs, 'cos you're only dropping about 5% off the voltage at most.

You should ditch the wirewounds anyway, so I advocate you try 1k first and if it doesn't work for you, go smaller.

You're right, it should be so low that it wouldn't effect the volume. In retrospect, I think that even unmodded that the 5150 gets about as much headroom that is possible with a 6L6 because they are already at maximum safe plate voltage. Choking the screen would bring the headroom to more stable 6L6 amps in the market.

I will see what the DC resistance of the primary to center tap is, depending on its value go from there. Still I am thinking adding two 2.7K5W Metal Film resistors for each tube in place of the ceramic wirewound. Most likely the voltage won't be too low to where I would have to lower the values, considering my screen voltage last time I checked was identical to the plate voltage (I was getting 498v on the screen with respect to ground and 499-500 on the plate) so I have a lot of voltage that I can sacrifice, so I think regardless of the mod, I will most likely stick with 1.35K on the screen.

 

[sprack]

^Damn that is pretty creative, why didn't I ever thing of that? How did it change the tone if any at all. I imagine that it had to of changed the tone a bit considering that you are eliminating the inductance to the screens. Are You running stock bias or has it been modded? And WTF is that burned out node between the two far right tubes did you burn something out?

Don't have an answer for that yet. I need to drill the mount holes for the choke tonite. I found a deal on a MM multi-choke so I can dick with what inductance works best. Also if you're installing a choke you may want to move the standby switch. Read this.

As the for the burn. It was like that from the prior owner. Looks like he smoked the molex connector and then pulled the pins for some reason. He had the powerboard hard wired to the main and I pulled it to put some quick connects on as well as replace the screen grids resistors. I splurged, spent $12 and I'm going through replacing all the CF resistors with MF (higher wattage, higher voltage, better precision). I know it's probably a useless exercise in using mojo parts.

 

[PaoloJM]

As soon as I did the bias mod to mine I cranked the bias to 42mA and I liked it that way. I know a lot of guys in the forum don't like going over 32mA, but personally, I like to stay clear the hell away from crossover, and 42mA seems to work great. I just hope beefing up the SGRs is not too much to kill the usable headroom and thus volume. I am hitting saturation at about 3.5 which is typical rehearsal volume, I really wouldn't want to get into saturation sooner.

No, you're not.
The 5150 power amp is very clean and does not saturate until around 8 or 9 on the post.
The full tone of the amp does not bloom or come alive until 3 and above, but the power amp is not saturating at this point.

Increasing the SGRs, within reason (1k works very well), will not reduce headroom or power, take it from someone who has done this a number of times.

 

[TheWinterSnow]

No, you're not.
The 5150 power amp is very clean and does not saturate until around 8 or 9 on the post.
The full tone of the amp does not bloom or come alive until 3 and above, but the power amp is not saturating at this point.

I know what poweramp saturation sounds like. With 4 75w speakers at 3.5 the amp begins to, compress a bit, at 4 you begin to hear more gain as if you turned up the pre gain knob for more distortion. At 5 you being to hear mild crunch, the highs are attenuated greatly and the bass starts to get looser but decrease at the same time. At 6, moderate additional crunch is heard and by 7, most of the highs are gone, and the bass is slowly falling apart. By 8, the amp is in a fair amount of total saturation, sounding as if the amp is about to melt down.

See the difference in volume from 4.5 to 8 is minuscule, tiny and laughable at most, its as loud as the amp is going to get, going beyond 4.5 does not make the amp louder, it just add more total distortion and compression. There is no way that speaker distortion could or would compress the volume to that extreme without a massive amount of audible speaker distortion, and even if it did, I highly doubt that 4 75W speakers with 97db sensitivity would begin to give out with the amp on 3.5.

I did this one day during band practice when we where on break, I chugged some open chords and brought up my post volume, at 4.5 the amp was at its maximum volume, and going beyond that added nothing more than additional distortion as if I was turning up my pre gain, but the added gain was looser and warmer as the amp sounded like it was slowly on the verge of a total meltdown by 7, which there was so much amp distortion that it was no longer musical. The speakers where just beginning to have audible distortion, but the cone excursion was still minimal. The resulting tone sounded as if I had taken recorded clip of the amp on 2, then added either a limiter or a clipper with a 100% softness, clipping the hell out of the signal by a few db. There was the tone of massive tube compression, which cannot be mistaken for speaker distortion as they are two entirely different beasts.

I have pushed solid state amps until they begin to distort the speakers, and I have pushed tube amps until they being to compress and saturate, anyone who has done the same can say the two are although very similar sounding, yet so far away. I know that with my setup, the amp begins to compress at 3.5-4, and after 5 the volume does not increase, you just get more total saturation and more tube compression.

 

[Omega_Void]

How high you have to turn the Post knob to get powertube overdrive depends on what level is coming out of the preamp, and applied NFB.

Different dial settings in the tonestack and the Presence and Resonance will make a big difference to that. Not mention the big discontinuity in commercial audio pot taper from being two or three straight elements instead of a proper logarithmic curve. And probably 20% part tolerance for carbon pots. A ~10dB difference isn't hard to imagine, and that's the difference from 4 to 8 on a log pot.

It's not really valid to make a sweeping statement that turning the Post to "x" will get the output saturating.

 

[TheWinterSnow]

How high you have to turn the Post knob to get powertube overdrive depends on what level is coming out of the preamp, and applied NFB.

Different dial settings in the tonestack and the Presence and Resonance will make a big difference to that. Not mention the big discontinuity in commercial audio pot taper from being two or three straight elements instead of a proper logarithmic curve. And probably 20% part tolerance for carbon pots. A ~10dB difference isn't hard to imagine, and that's the difference from 4 to 8 on a log pot.

It's not really valid to make a sweeping statement that turning the Post to "x" will get the output saturating.

Very true, due to the nature of using a differential amplifier as a way of phase inversion in conjunction with a negative feedback, the reactance of the filtering in the feedback will change the gain factor and therefore the amount of current that is going to be received by the tubes, as well as the frequencies that will saturate harder both in the phase inverter and the power tubes. On a side note it was Wolfe that liked 12AT7s in the phase inverter slot because he liked the cleaner headroom going to the tubes, saying that it helps keep the amps sounding defined at higher levels, and I highly agree.

 

[endofall]

TheWinterSnow, Omega Void, Sprak, and PaoloJM, you guys seem to understand more about what is going on inside a 5150, earlier in this thread someone mentioned that they believed it was R95 that was replaced with the cap/pot set up for the mid sweep mod, could one of you guys comment on this. Thanks for any help!

 

[Omega_Void]

It's the resistor that's in parallel with the treble pot.

On this schematic I found it's labelled R22. Tonestack is toward the top right of the image.

 

[TheWinterSnow]

Unlike Wolfe, I really DO NOT want to change the tonality of my 5150. Rather, I would like to make the amp more usable, mild things like take out the fizz, make the gain more musical. Same thing goes with the tonestack. I would like to get a mid sweep on the amp as well, however, the tone stack is one of the few things I know nothing about (that and the 5150's clamping circuit and the way the tone stack is driven). From what I have done with tone stack calculators, a variable pot on R22 does sweep the mid frequency but the lower the mid frequency, the the bass is attenuated, and it become quite dramatic. Now I don't know how the cap into the pot changes things, but considering that Wolfe mentioned that they would be in series, I can't say that I think it would do much.

Regardless when the the information comes in about the mid sweep, I will still throw a switch to go back and forth between the sweep mod and the stock tone stack, to give the amp more versatility, but still be able to go back to stock settings if I ever want to.

EDIT: From what Wolfe was talking about on the first page about the midsweep and from what Omega pointed out also, I have rewritten the schematic for the tone stack.

Mid Sweep:

And My mod that allows you to go back to the stock tone stack:

Also if anyone is curious I can post screen shots of the tone stack simulator, showing how using the sweep knob effects the rest of the tone stack. Its pretty interesting what changing the resistance of R22 does the both the mids and the bass.