bassman metal film build

[Garycbet]

has anyone built a bassman using metal film resistors.  was there a big difference in sound? if it was different how would you describe the difference?

i couldn't find this kind of advice on the forum or any other.  most folks just tell me not to.  would love to hear some opinions from this forum.

thanks
 

[uki]

Here some interesting points about MF resistors.

I would suggest you use all flameproof metal film resistors everywhere.
Safer, quieter, less prone to drift and failure
I don't see the advantage of carbon comp resistors myself.
I try to never use them unless I need one while I'm building and it happens to be all I have or it's all I can get in a particular value.
In my opinion, that's on par with orientating capacitors

That's just my opinion on Carbon comp resistors, as Willabe suggested do some testing and decide for yourself
It really is a debatable point and like many things there's a lot of opinions on all sides of the argument, like capacitors, tubes, wire, speaker magnets, etc etc etc.
I tend to fall into the , 'if it can't really be proven one way or the other then it might not be a factor', camp

Now if you had a small amount of floor noise and had use all MF resistors and wanted to get dead quiet maybe then it would be something to try.

I read somewhere , carbon composition resistors do good at high voltages, but where the voltage isn't that high, they are noisy and don't do much in tone terms at those spots, so something else could be used, like the MF as you mentioned in those noise sections of the circuit, how do I find where to use the MF in the circuit?

Thanks Uki. 

The places to maybe use CC R's is where there's a high DCV, plate R's, CF R's. Grid return/grid leak R's and K R's don't have high DCV on them so no advantage to using CC's in those places.

You'll have to try them for yourself to see if you think they make a difference in the tone/sound. 

[Garycbet]

thanks for compiling these posts UKI.  i would not generally specify carbon comps in much of anything.  there were some instances where my training fell into contradiction with real world results.  my instinct is that there should be no difference other than noise and longevity (huge considerations actually). So many swear that there is a real difference from one component to the next that i felt getting the opinion of someone who has been in a position to try both would be wise.

[SoundmasterG]

Here are some links for you that you may find illuminating.

Greg


http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm


http://www.aikenamps.com/index.php/resistor-types-does-it-matter


http://www.resistorguide.com/carbon-composition-resistor/

[Garycbet]

hi all.  everything is going well. any advice on where and how to to mount the filter caps.  im looking at picking up the bassman cap can but not sure where this sets in the chassis that doug sells.  any pics or advice. tweed cap board a good idea?

thanks

[uki]

Hey there, check out this thread it is for another bassman, not sure if it is the same model as yours, but may help out with some clues. There are lots of pics !

[Ed_Chambley]

I have used both and cannot tell any difference except the MF make less noise.  I use 1 watt MF resistors unless someone wants something different.  There is a measurable distortion, but I cannot hear it.

I have a theory, which means nothing really.  Here it is.  All of the guys like me who grew up playing amps with Carbon Comp resistors got used to the floor noise they make and this noise stirs memories from the past.  Memories are powerful.  When I hear the sound coming from CC resistors it does remind me more of Vintage Gear so therefore I could be swayed into thinking the tone is more vintage.

Then what is vintage tone?  Is it a nice tone while playing and a smudge of hum when idle?

In a band situation it really makes no difference to me, but now days I can build with very little floor noise and prefer this.  Some of the amp Gurus will say to use them as plate resistors and not a bad place to put them if you like the sound of bacon frying.

Seriously use whatever you prefer, but you will not find them inside of most newer handwired amps except those who are truly trying to recreate a clone.

Dave Funk prefers them and he has forgotten more about amps than I know.

[Garycbet]

I have used both and cannot tell any difference except the MF make less noise.  I use 1 watt MF resistors unless someone wants something different.  There is a measurable distortion, but I cannot hear it.

I have a theory, which means nothing really.  Here it is.  All of the guys like me who grew up playing amps with Carbon Comp resistors got used to the floor noise they make and this noise stirs memories from the past.  Memories are powerful.  When I hear the sound coming from CC resistors it does remind me more of Vintage Gear so therefore I could be swayed into thinking the tone is more vintage.

Then what is vintage tone?  Is it a nice tone while playing and a smudge of hum when idle?

In a band situation it really makes no difference to me, but now days I can build with very little floor noise and prefer this.  Some of the amp Gurus will say to use them as plate resistors and not a bad place to put them if you like the sound of bacon frying.

Seriously use whatever you prefer, but you will not find them inside of most newer handwired amps except those who are truly trying to recreate a clone.

Dave Funk prefers them and he has forgotten more about amps than I know.

thanks Ed.

OK so that amp is built ill be posting pics soon.  I noticed that the chassis is screen printed differently than the layout Doug has.  Meh not really a problem at all.  just caused a bit more rechecking to see that I didn't mess the orientation up.

I know i didn't use the correct grounding but dont have issues so far.  the OT ground runs back to the PT ground lug and so does the ground of for the speaker out.  I also connected the pot buss to this point as well.  My training says this is wrong but seems fine?  any opinions on this or better suggestions.

[Garycbet]

ive attached some pics from the phone to start. 

[Paul1453]

All my metal film resistors are 1% tolerance.  I like that. 

Besides the flameproof, noise, and drift issues with the less precise CC resistors.

I tend to try to only use my higher watt CC resistors in the PS section now.   

[Garycbet]

Thanks Paul how do you feel about the ground question?
 

[Willabe]

Maybe this will help;

http://www.valvewizard.co.uk/Grounding.html

[Paul1453]

Thanks Paul how do you feel about the ground question?
 

I haven't made a Bassman yet.

But many very experienced builders here have.
So they will be able to give you much better advice than I could.   

Best regards,
Paul

[Garycbet]

  ok I checked rechecked and checked again walked away then checked again.  I powered up the amp dim bulb. checked voltages and all seemed well.  plugged back into wall power, set bias then grabbed my guitar.  very very low gentle hum from the speakers.  the guitar volume was very low.  at 12 o'clock there strum a big chord and there was audible sound but very very low.     its been a long day.  ill have to figure out what I messed up tomorrow.

Very frustrated.

[HotBluePlates]

...  the guitar volume was very low.  at 12 o'clock there strum a big chord and there was audible sound but very very low.     its been a long day.  ill have to figure out what I messed up tomorrow. ...

Picture of the completed chassis? Voltages at all tube pins?

[Garycbet]

Picture of the completed chassis? Voltages at all tube pins?
[/quote]

ill send the pics in a bit. all heater voltages present at 6.9 vac no tubes other than recto installed.

with respect to ground
v1      p1 548.7       p2-p5 0v       p6 547.9   p7-p9 0v

v2      p1 547          p3-p5 0v       p6 552.7   p7  547.1   p8-p9 0v

v3      p1 553          p3-p5 0v       p6  547.2  p7  547.2   p8-p9 0v

v4      p1-p2 0v       p3 556.0       p4  555.4  p5  -56.8   p6 -56.85  p7-p8  0v

v5      p1-p2  0v      p3 555.3       p4  555.4  p5  -56.6   p6 -56.6    p7-p8  0

schematic out and seeing how these fit in.

[eleventeen]

"all heater voltages present at 6.9 vac no tubes other than recto installed."


That's no good for the kind of remote diagnosis being offered. Nothing will be drawing any current, thus no resistor will produce any voltage drop. Conceivably you could even blow some ecaps on the C & D nodes as their voltage ratings are below those of the caps that "see" the big B+ used to supply the output section.


I gather from your prior post you had the tubes in and had a hum/low volume condition. No smoke, no flames, no electro-stink?

Get those tubes in there and repeat the pin by pin voltage measurements and we'll get you going.

[Garycbet]

ok  tubes installed.

v1
p1 139v      p2 0v      p3 1.7v       p6 172.7v      p7 0v           p8 2.62

v2
p1 186v      p2 0v      p3 1.32v     p6 342.6v        p7 185 v      p8 186.7

v3
p1 420v      p2 31.96v    p3 69.1v     p6 315.3v     p7 56.23v      p8 69.0v

v4
p3 476.5v      p4 475.5v       p5 -56.03v    p6  -56.03     

v5
p3 476.5v      p4 467.6v        p5 -56.03v   p6  -56.06v


all other pins read zero with respect to ground. 

[Garycbet]

really don't like the speaker jacks.  messy work  im going to chase down the wires and make sure that I made all the connections just to be sure once again.  I fear that I may have a couple of cold solder joints as well.  its really tight working in the bassman chassis.  this may be the last tweed style i do.

[eleventeen]

I myself am not much of a fan of Tweed amps and I have almost no experience with them. I know they are tough to get your iron and tools into. However, I would get rid of those 4 RCA jacks going to the speakers and wire 1 qty 1/4" jack and jumper the speakers externally. Personally...I don't consider an RCA jack an appropriate connector for a speaker. Yes, I know, that's how Leo did it and here on perhaps the most famous amp of all time. It's a 50 watt amp, those speakers each get 10 or so watts. An RCA is not a 10 watt connector. (Arguably, a 1/4" is not a 50-watt connector)


If you are going to go on a cold solder joint hunt (should not let those go while building, even 1 of them, without immediately correcting them) I would go get some liquid flux and use that when reheating the joints. You built this out of all brand new parts, there is no good reason why you should have cold solder joints. Except maybe you didn't clean your iron while soldering, and/or modern solder is sometimes lead-free and IMO it does not wet the joint as well as solder used to.

[Garycbet]

Thanks eleven.   I'm fairly certain that I do not have a cold solder joint. But I've got to check anyway. I may have put a wire in the wrong place.  In either case I'm in for six weeks of troubleshooting.  Lol.

[HotBluePlates]

-56v of bias on pin 5 of the output tubes seems awfully high. What is your tube current across the 1Ω resistors?

Separately, you have different voltages for each grid on the long-tail inverter... They should be essentially the same voltage.

Eyeball those 2 things and tell us what you find out.

[Garycbet]

33.7  and 33.0  ma of current at the 1 ohm resistors.  good catch on the Pin 4 voltages.  but a remeasure shows that they are within .1 volts of one another.  478 and 477.9.  pretty consistent  I measured twice afterwords i get the same readings.

[Garycbet]

What sort of symtems would result from a disconnected feedback wire or if this were shorted to ground?  Hoping those of you with more experience could help.

[HotBluePlates]

33.7  and 33.0  ma of current at the 1 ohm resistors. ...

Right now you have ~475v on the output tube plates. Try bumping the idle current up to 36-37mA. Perhaps the plate voltage will come down a touch as well.

Separately, you have different voltages for each grid on the long-tail inverter... They should be essentially the same voltage.

By this, I was talking about pins 2 & 7 of V3. They should be the same d.c. voltage.

What sort of symtems would result from a disconnected feedback wire or if this were shorted to ground?

A disconnected feedback wire should pose no problem to the amp. If fact, since the feedback connects to one of the phase inverter grids (through a blocking cap), I was gonna suggest disconnecting it for a quick test anyway.

[Garycbet]

Ok I walked away for a couple of days.  Came back and fixed the RCA jacks that I hated and there it was.  Works perfectly.  Super quiet with my humbuckers.  Bit of noise with the strat but that's what I expect.  Could not play with more volume because the kids were asleep.  Here is a link to some low volume fiddling.
https://youtu.be/4g3_ZxkPBBE

[HotBluePlates]

Congrats! 

I had wondered about those jacks, but had no good way to advise you to test them. I'm guessing there was a cold solder joint or a solder blob somewhere?

[Garycbet]

Congrats! 

I'm guessing there was a cold solder joint or a solder blob somewhere?

That's completely possible....  My solder technique is well above novice level but every one makes mistakes now and then.  Yep they were very ugly joints.  The real problem was waiting till last to put them in.  When there were plenty of wires in the way.  Lesson learned.  In any case.