70s build - some questions (new - added some clips)

[Bob-I]

The only thing I haven't been able to do is really get it to sing, it seems on the verge but just wont quite give up the goods.
Bill

I hear what you're saying. It's like it's almost there but just won't go over the top. IME this can be due to the clean side not giving you enough gain. I'd make sure that the first gain is run at at least 1:00 with a standard audio taper pot. I believe the OD entrance is fixed but I'm not sure about the V2b gain.

I'll look at the #40 schem and see what I think.

From what I hear this amp has great potential. The crunch tones are excellent, but like you said, it just needs a tad more singing tone.

[wjdunham]

if you have an audio taper treble pot you'll have to turn it up quite a bit to hear a mid boost-

I have a 250K audio taper for the treble - and you're right, if I turn it up well past half way I hear the mid boost much more. The tone stack currently has all audio taper pots, 250k treble, 250k mid, 500k bass.

On the gain side, I do have the clean set to about 1:00, OD drive set about the same - I try to set them just below where it starts to get muddy and fuzzy, then crank the master to get the heavy crunch. I thought maybe that guitar was part of the problem, the action is set a little on the low side for a strat, but it's still doesn't quite sing with any of the guitars I tried.

[Max]

I put an AT7 in late last night, too late to crank 'er up. I checked the voltages and it dropped the PI plates to something in the 225 range, and V1 down to 160. I haven't done any research yet on how to really bias with a 12AT7. Should I up the cathode resistor some to get the idle current down a bit? Would I also have to adjust the plate resistors? I didn't really want to mess with the PI too much as I'm told it's an important part of the sound and feel of these amps. At very low volume I didn't really notice a difference between the two tubes types.

As far as I remember Alexander specified AT7 on the chassis of the second and third generation "pre classic" amps (from around #015 up to around #70).

But all players who use one of these amps that I personally know (myself too) liked their OD sounds more with a 7025 PI tube. For my hearing this was only a matter of taste, different sounds of course but both fine. I'll give an AT7 a try on next occasion and report. It's years now, that I tried an AT7 in one of these amps.

For input and OD I always liked best NOS Mullard ECC83 and for PI NOS GE 7025 JAN.

Cheers

Max

[Bob-I]

Bill,

Looking over the #40 schematic, that amp should have a ton of gain.

Post your settings, perhaps the lack of a seperate OD level has you running the gains incorrectly for the singing tone you're looking for.

[Funkalicousgroove]

There is a noticable volume drop using a 7025 PI, whereas the clean is not near loud enough!

Dumbles used 30% taper pots on the Log pots, modern pots are 10%-15%, turn it up!

[Max]

There is a noticable volume drop using a 7025 PI, whereas the clean is not near loud enough!

Me and no one else I know who played one of these amps using a 7025 for PI regulary in the studio or live ever had any problems in practical use like not enough overall loudness or not loud enough clean sounds.
It's only some weeks one of these (a 50 Watt this one) with a 7025 PI went on the bench and measured at 50 clean watts (at somewhere between 1 and 3% THD as far as I remember now) just as it should after all I know.

Just my personal experience. Others may have other amps and may make other experiences.

Cheers

Max

[Funkalicousgroove]

Max,

We are friends, and I do respect you, BUT:

At what frequency?
at what amplitude?
What equippment was used to measure the THD?

Typically one would use about 1/2v peak to peak(average guitar pickup) @ 1K-2K and have all the knobs maxed on the amp, and monitor the output to a pureley resistive load with an oscilloscope/spectrum analyzer/multimeter. At those conditions I really DOUBT that amp made a clean 50 Watts at the onset of clipping with only 3% thd.

Now, Increasing the input signal to achieve the voltage at the output is not a real world comparison if the voltage surpasses that of a guitar pickup, ie, it may make 50 watts with an ENORMOUS input signal, but that's not what would generally be plugged into it.

Alternativeley, one could plug signal directly into the input of the phase inverter, but it is only a fair comparison if the voltage is equal to what would be coming out of the last stage of the preamp at the above conditions.

I hate to be one to say this, but your last post may not contain 100% accurate information.

[erwin_ve]

There is a noticable volume drop using a 7025 PI, whereas the clean is not near loud enough!

Why is that?
To my knowledge 7025 tubes can also have a gain of 100%

[Max]

Max,

We are friends, and I do respect you, BUT:

At what frequency?
at what amplitude?
What equippment was used to measure the THD?

Typically one would use about 1/2v peak to peak(average guitar pickup) @ 1K-2K and have all the knobs maxed on the amp, and monitor the output to a pureley resistive load with an oscilloscope/spectrum analyzer/multimeter. At those conditions I really DOUBT that amp made a
clean 50 Watts at the onset of clipping with only 3% thd.

Now, Increasing the input signal to achieve the voltage at the output is not a real world comparison if the voltage surpasses that of a guitar pickup, ie, it may make 50 watts with an ENORMOUS input signal, but that's not what would generally be plugged into it.

Alternativeley, one could plug signal directly into the input of the phase inverter, but it is only a fair comparison if the voltage is equal to what would be coming out of the last stage of the preamp at the above conditions.

I hate to be one to say this, but your last post may not contain 100% accurate information.

Hi Brandon,

I think I understand what you say and I will ask the tech who did the service and report.

You are right, was not very accurate:

I just would report my own impressions and those of some others who use(d) these kind of amps in regard of daily practice, from a gigging musicians point of view.

The measurement results I only reported in the sense of: "tech said everything working fine with new 7025 PI tube and new 7581A power tubes". But I really don't know - without asking him - in what detailed way he made sure that the amp is working fine with these tubes. When I've talked to him, I'll report the results.

Perhaps the amps you and I are talking about are different in some way and this difference is the reason of our different observations in regard of possible PI tubes?

I always wondered, why me and some of my friends didn't like the AT7 as usually it has always made sense to follow Alexander's recommendations and what he specified for his amps. Perhaps it is only a matter of my (bad?) personal taste?

Just as I write this I seem to remember, that I know of one or two ODRs where I liked the AT7 better and the 7025 didn't deliver as good results as PI (for my ears).

What I reported "from the bench" may even have the reason that the tech thinks: "O.K. if you fools like the 7025 better, do it, the amp will not explode."

O.K. - no more speculation - I'll ask him about this.

Thanks for the criticism - open talk between friends is IMHO one of the best ways of learning I know. This is always welcomed and the more if it comes from you.

Have a great time and all the best

Max

[wjdunham]

On the general subject of gain - once I reduced the amount of GNFB, the volume of the amp is as I would expect - at least based on where I have to set the Master - anything past 10 oclock is starting to get plenty loud. This is with the FET input and 3db of gain dialed into the FET trimmer. The normal input is plenty loud as well, but I just have to crank the volume and OD drive to get the saturation that I like. Clean volume with either input is not an issue.

I was thinking I need to play with the tone stack to get the pots values and tapers closer to where they should be for this type of amp, right now I have all audio tapers, I was going to change the treble to liner and up the value from 250K to 500K. There is a lot of information on these early amps scattered around the forum and I'm starting to piece it all together, sounds like #40 schematic still has a lot of inaccuracies.

[Bob-I]

On the general subject of gain - once I reduced the amount of GNFB

Let's not get confused here. GMFB has a big effect on the amp's tone and dynamics but it has nothing to do with the pramp gain structure. The singing sustain you're looking for is strictly pre-amp gain. Get that preamp to sing and then tweak the GNFB.

[Max]

How did my clip sound in your ears relative to others you have heard?
Thanks,
Bill

Must first get my headphones back; the speakers of my computer are crap. I don't believe in judging musical instruments by listening to mp3 clips anyway, but I'll tell you my impressions after listening.

Max

[wjdunham]

I agree Bob, I reduced the GNFB for two reasons - I generally like the looser feel of an amp with less feedback, and second to get a little more volume from the poweramp, I had some other problems related to the connectors/switches that was really causing the volume problem, and I have not gone back to the original values. I agree that it's one of the fine tune points after preamp is singing.

So there are still a couple of more points to experiment with, neither of which I've had a chance to try yet - tone stack pot values and tapers, and the output of V2 - I have 180K into a 100KL Ratio Pot, I could ding the ratio pot and use the 1M/180K configuration shown in the picture. Also could play with the OD entrance, which is probably where most of the signal is shunted. Some conflicting opinions on what's the best thing to use there - Brandon's amp has it and it definitely sings like a bird. I have his values in mine right now.

I didn't see any opinions on whether it's OK to just swap in a 12AT7 and leave it biased such that that it's drawing a lot more current than the 12AX7 (thus pulling all the voltages down) or adjust the plates/cathode to put it at a different point. They are pretty different animals and I though you had to change the bias point to really take advantage of the differences?
Bill

[Funkalicousgroove]

Max,

7581A tubes also make a difference, they are a ruggedized Military spec tube, and I believe they are actually a 35 Watt tube(sylvania version) so they "Should" make more power.

Fat Willie claims that he gets Derek trucks' Super 6 to make 120 clean watts using 7581's and 465v on the plates- Same amp with new off the shelf tubes and original PT makes 83 watts-

[talbany]

Max,

7581A tubes also make a difference, they are a ruggedized Military spec tube, and I believe they are actually a 35 Watt tube(sylvania version) so they "Should" make more power.

Fat Willie claims that he gets Derek trucks' Super 6 to make 120 clean watts using 7581's and 465v on the plates- Same amp with new off the shelf tubes and original PT makes 83 watts-

These were designed to compete with the KT-66's at the time.. At least they were advertised as the American version of the KT.. This always confused me as they were not kinkless tetrode..I've tried several and didn't particularity care for their breakup quality it had (at least in a 50 watter a 100 might be better)...I still much prefer the Sylvania 6L's or STR's much smoother break to them..Not much magic there..At least I didn't think so..Amps I've tried them in were..JTM45 shared cathode
Tweed Pro..Cathode bias
Classic 50w Skyliner
Regulator.. My Marshall 59 variant..

Stevie tried them in his Vibroverb for a spell but didn't really warm up to them either...

Tony VVT