new here...a few questions bout my 1st amp

[iknowjohnny]

I've tried everything imaginable in the TS.
As to gain, i have tried removing stages and tried juggling stages around. A single stage is the same bass wise. No matter how much or little gain or which tube/side(s) it's coming from the bass stays the same.Even tried bypassing V2 with the cathode follower and tone stack altogether and going right in the PI from V1.

[John_P_WI]

Sounds like you have tried about everything.

Input jack is wired correctly? Grid stop on V1 is in place? You can always try increasing it.

I have had success in using lower value vol pots or paralleling a 500k resistors etc to take some gain out, ultimately tightening the bass.

Another thing to watch is the pickup height of the guitar, this can lead to woofy bass if it is too high/ close to the strings.

[iknowjohnny]

yeah, i think i'm pretty much done here. I HAVE tried everything, almost at least. I've gone so far as to try things like unmounting the OT and moving it around and things like that. I apparently have a bum amp and nothing seems to be to blame. I'd happily trash it right now, but i want to build a bigger amp and if i can't make this happen i'll be too gunshy to try again.

[John_P_WI]

Don't give up on amp building. We have all (those who have been at it long enough) built amps that didn't live up to expectations. I've shelved amps before, only to come back to them years later.

My guess is that someone local with a fresh set of eyes, a scope and signal generator could sort it out in a very short time.

Good luck,

John

[iknowjohnny]

Thanks for the encouragment. But i just don't understand how a known good scheme (i say this because it started as a bone stock 18 watt TMB) who's parts have all been checked, double checked, and checked again and morhed into 30 different circuits can never be rid of these issues. When theres nothing left to do, how do you explain it? thats why i just don't know i can go thru this again. It's be a horrible experience.

[Ears]

Assuming I understand correctly that you have no issue with the level of bass, only in that it is "too loose". It's straight forward to indentify using a scope and signal generator where if you have have losses.
Descriptions such as "tight" are common to speaker damping issues and impedance matching. I'd now look at the power amp and specifically output transformer speaker interface.
So if you haven't already perhaps it's time to get a scope and/or check the output transformer, substitute it or remove it for bench tests.

[iknowjohnny]

The OT is a GDS/heyboer 18 watt. I have a heyboer OT that was made for my peavey classic 30 which has 1/2 the primary impedence of course, but i tried it and played with the secondary taps and no difference. I've done that both when it was EL84's and after i coverted to 6V6. I also played with cathode values and screen values a lot, so i don't know where else to go. Granted, other speakers i've tried have been tighter than the EV12L, which i'm using because it my all time fav speaker. But not perfect either. the issue is still there, just less bothersome. But i have used this 12L with every amp i've owned in the last 20-25 years and it's never once had any sort of bass issue. Not even close. so it's gotta be in the amp. i just can't imagine where else to look. And yes, the bass is not really too much, just fuzzy and loose. One word that strike a chord with me when thinking about how it sounds is "cardboard" if that means anything to you.

Anyways, i have access to a scope but don't know if it even works or how to use it. Not even sure if the probes are there.

[Ears]

I assumed from your own troubleshooting that the problem appears before V1 and/or after the PI.

But at risk of stating stuff you have considered here is an extract from Valve Wizard on cathode caps in particular electrolytics.

Because 22μF is a fairly large value of
capacitance we will probably be forced to use an
electrolytic capacitor. The cathode operates at only a
couple of volts so many designs use a 25V
component. However, it is an unfortunate property of
electrolytic capacitors that they have a limited
working life, relatively high ESR*, poor high
frequency response and, when operated at polarising
voltages far below their maximum working voltage,
poor low frequency response too. This can produce
noticeable and unpleasant distortion, often manifesting
as a ‘wooliness’ or ‘sluggishness’ on low notes. A
further problem with using a large cathode bypass
capacitor is that the stage may have too much low
frequency amplification. If the stage begins to
overdrive the following stage, too much low frequency content can cause
blocking distortion, which is most undesirable. This is most important in bass
amps of course (often it will pass unnoticed in a guitar amp), but in any amplifier
using several gain stages the effects will be cumulative, so it is worth consideringan alternative. There is a number of solutions:
• Use a lower voltage component, say 6.3V. This may improve bass
response somewhat, but low voltage electrolytics often have extremely
poor tolerance, which may or may not spoil a carefully designed stage.
• Use a non-polarised electrolytic. These capacitors do not require any
polarising voltage and give consistently improved results when
compared with ordinary electrolytics, but they may not be available in a
wide range of values.
• Use a tantalum capacitor. Tantalum capacitors generally give much
better performance than ordinary aluminium electrolytics, though they
are more expensive.
• Use a non-electrolytic capacitor, such as a poly’ or silvered mica type.
These give the best sounding results, though are not readily available in
values much larger than 1μF, but for guitar this is unlikely to pose a
problem.
Most early amplifiers used a fairly large cathode bypass capacitor since they
used a limited number of gain stages and it was assumed that they would not be
overdriven (the Marshall 50W Master Volume used 330μF in one channel!). As
usage evolved and guitar pickups became ever more powerful (a modern highoutput
humbucker can deliver around 4V p-p in the neck position!), it became
important to maintain a tight and defined bass response even when overdriving,
and ‘bright’ and ‘top boost’ tones became popular too. As a result, more
amplifiers began using smaller values of cathode bypass capacitor. The most
popular, well-designed amplifiers of recent times almost exclusively use partial
bypassing, after much experimentation no doubt, to encourage glassy, sustained
high-note distortion with un-muddied bass tones. Reducing over-sized cathode
bypass capacitors is a cheap and safe way of experimenting with, and improving
the tone and transient response of an existing amp, particularly one that sounds
too bassy and muddy. What is more, smaller values of capacitance allow much
higher quality, better tolerance, non-electrolytic capacitors to be used readily.
Fig. 1.19 shows the frequency response of the same stage with different possible
values of Ck. The stage has an obvious ‘shelving’ effect- boosting higher
frequencies. Reducing the size of the capacitor shifts the ‘shelf’ up the frequency
scale, without changing its basic form. The dot on each plot indicates the
transition frequency for that value of Ck, which can be found using formula VI.

[iknowjohnny]

Thanks for that. A very interesting read. I don't think it applies to me because i have been thru a lot in the cathodes of the preamp. i've used non polarized 1uf caps and no caps at all on the second stage and i can't imagine that would be the issue. I've even has a .1uf of very small voltage on the 1st stage and no cap on the second. but a good read none the less, and something i'll definatlely keep in mind if i build another amp.

[CaseyJones]

I've tried everything imaginable in the TS.
As to gain, i have tried removing stages and tried juggling stages around. A single stage is the same bass wise. No matter how much or little gain or which tube/side(s) it's coming from the bass stays the same.Even tried bypassing V2 with the cathode follower and tone stack altogether and going right in the PI from V1.

O.k., I'm a latecomer to this thread. I'll toss in a couple ideas...

First, K.I.S.S.. That stands for "Keep It Simple, Stupid." It ain't simple and we ain't stupid but it sometimes helps to strip it down to the basics and figure out what you have.

First simplification: Remove that PPIMV. Smashing your preamp into the brick wall of a PPIMV usually sounds ugly. Some people like thin buzzy sounding distortion, if you ain't one of 'em then you don't want a master volume. Besides... master volumes are for pussies!

Once it's tight with low gain add gain stages. You want to bleed off a lot of gain interstage with voltage dividers. Too much gain in the wrong place is just gonna sound ugly. You can't kick the shit outta a 12AX7 then layer on the Band-Aids and expect it to tighten up. But you tried that and it doesn't help. So:

Look at your P.I.. Are you using 18 watt Marshall values or are you using 5F6/JTM45 values? You now have 6V6s in there, they want to see more excitation at the grids than EL84s. Make it so. If you have wimpy drive to your 6V6s it's gonna sound wimpy. Look at Mark Huss's 6V6 Plexi on his site for reference. Just because you have massive gain at your preamp doesn't mean that you have adequate drive at your P.A. tubes or that you don't have some screwy impedance thing goin' on there. PPIMV = screwy impedance thing BTW. Copy the high powered Marshall P.I., you seen one you seen 'em all!

It may well be that your high gain preamp design is sound but somewhere along the signal path it all goes wrong.

It's a matter of balancing gain, it's all a balancing act. Too much in the wrong place sounds ugly, too little in the right place sounds ugly.

[dobbhill]

What about layout/lead dress?
My favorite tone nightmare.........
D

[CaseyJones]

I Put in 6V6's in place of the EL84's and cascaded the pre. i have spent endless hours fighting certain tonal issues and hoped i might get some better results here than i've gotten elsewhere. The pre is very simple really. 2 stages in the 1st triode with grid blockers on all stages, 1 meg gain pot, voltage divider at stage 2, then into the V2 tube as the typical cathode follower and marshall tone stack. all couplers ate .022 mallories. Tried endless cathode R and cap arrays on both sides of v1. The power amp has a 1k shared screen, (want to change to 2 470's but out of them) a 265ohm cathode R with a bypass cap at 47uf, but have tried all the way up to 2200uf.

Ok, i know there is no simple answer to any of this...

O.k., I know I come off as my usual prick(ly) self but I actually put some thought into this, bear with me:

I was out at the races last night. It sounds to me like you been workin' on yer carburetor when you got no spark. You made endless preamp tweaks, paid no attention to the PI and minimal attention to the PA.

In your plate to plate amp there are several ways to arrive at a low fidelity (no bass) in the power amp, look for the following:

Your amp may be oscillating wildly. The usual indicator of that is low voltage at the power supply, what's happenin' in that case is yer amp is workin' its ass off to amplify sumthin' you can't hear so it's got nuthin' left for what you can hear.

Grossly mismatched output tubes. If you have one side of your power amp doin' all the work and the other side just along for the ride it's gonna sound weak. Pull one 6V6 to see what I mean. If you're lucky that's the side of the output that's screwed up and it won't sound any different.

Screwed up output transformer. Heyboer's quality control is really good but I don't know where your OT has been so it's possible one side of yer OT is, how do I put this? ...fooked. And yes I've chased my tail on this every so often, gone around in circles then eventually jumpered in another OT just to see what it would do. If you've ever shorted a PA tube in yer amp it's possible to damage the OT and it won't just fix itself. If yer amp has ever been in a condition where it eats fuses it's possible to damage the OT and it won't just fix itself. If you're workin' on yer amp and you spark it up without speakers connected it's possible to damage the OT and it won't just fix itself.

There is in fact a simple answer. Go single ended. Seriously, if yer plate to plate setup is kickin' yer ass change over to SE. K.I.S.S.. You say 18 watts plate to plate is too much, thus the master volume. So... eliminate the PI, then buy a Hammond 125 CSE, DSE or ESE for various levels of overkill. Your high gain preamp easily has enough gain to drive a single EL34 or KT66, or you already own a pair of 6V6s so pick a good one and run that single-ended. Your Heyboer PT easily has enough juice to run one PA tube instead of two although the voltage will be on the low side for a KT66 or an EL34. No biggy, it's a brown sound application. If it ain't workin' strip it down until you get something that does work. Stripped down, there's less stuff in there to kick yer ass.

Finally, yer moment of Zen: Ken Fischer and H.A. Dumble among others managed to do what they did pre-internet, more or less lone mad scientists who actually brought the monster to life in the relative isolation of their own secret laboratories. Ponder that next time the answer proves elusive.