blocking Distortion? 5C3+5c1 custom amp

[redfoxblues]

good morning fellow amplifier pioneers

I have recently built an amp which is basically a 5c3 with a 5c1 front end. I changed the 6sj7 bias from grid leak to cathode bias. the problem I am having which is why I changed the bias on the 6sj7 is im getting what I believe is blocking distortion or grid blocking. it started with me monitoring the bias of the tubes while playing and noticing that the bias was dropping rapidly while playing the amp until they cut off. Changing the bias of the 6sj7 helped tremendously and I did so because of what I had heard on grid leak bias being bad for guitar amps. since I am still getting this behavior but not until the amp is turned up near max. at low volumes it behaves normally but around 75% up the bias starts to fall fast again. I installed grid resistors of 1K but it hasn't helped much. One interesting fact I have discovered is that this behavior does not happen when I measure the voltage at the grid while playing?

thanks in advance
Nate

[g1]

Post your schematic.

[Tom Phillips]

...One interesting fact I have discovered is that this behavior does not happen when I measure the voltage at the grid while playing...

Note that most common test meters will add a substantial load, compared to the grid input impedance, when you attach the test leads. This is a case where the act of measuring something affects the circuit performance because it changes the voltage you are trying to measure. Attaching your meter is probably the same as attaching a 1Meg resistor from the grid to ground. Better meters would have a 10Meg input impedance and higher end meters can be much much higher such as 10 G Ohms input impedance (or more) for the DC measurement function.

Anyway...I believe that's the reason for the behavior you observed.

Cheers,
Tom

[Chuck H]

If the grid voltage remains stable when measuring, how do you know the bias is shifting?!?

One interesting fact I have discovered is that this behavior does not happen when I measure the voltage at the grid while playing?

So how are you measuring the bias?

Also, you mention the bias of a preamp tube repeatedly, but I get the impression you're talking about the power tubes:

it started with me monitoring the bias of the tubes while playing and noticing that the bias was dropping rapidly while playing the amp until they cut off.

Can you clarify any of this?

[redfoxblues]

well I was thinking that the meter had some sort of effect on the circuit so thanks for clarifying that (tom) and my idle bias of my power tubes is -24 and the b+ is 282. This all adds up to be about 13w diss per 6v6s(jj). Chuck, I was measuring the grid voltage with my digital multimeter set to dc voltage and it was shifting from around zero to about -17. I only measured the grid after reading more on blocking distortion. All of this came out of me noticing the power tube bias falling rapidly while jamming. I have tried to figure it out by researching but I had to throw in the towel and call upon the knights of the vacuum tube to help me . The amplifier is like I said a mix between a 5c3 and 5c1 where the preamp of the 5c1 feeds into the volume pot from the 6sj7 into the 6sc7(6sl7 in my case) phase inverter into the power tubes. I read that the 6sj7 was grid leak bias and when I had it biased according to the 5c1 schematic it had blocking distortion way worse and the bias would not stay high enough to play the amp at a reasonable volume. I changed it to cathode bias and it got better but still persisted when the amp is near max. I had used 330k resistors on the phase inverter in place of the 250k as that was what I had at the time. I reduced these to 180k and it helped but didn't seem to take it all away. I attached a layout of what the amp looks like and forgive me I am schematically illiterate or maybe I have never given myself the time to connect the dots although its better than in the beginning I still find it easier to look at the layout. To top it all off lol I have developed some parasitic oscillation of some sort with volume maxed and it causes my b+ and all other voltages to wobble up and down. I guess this is what I get for going off the beaten path lol.

[MarkusBass]

Is pin #4 of the tube a grid? It seems to me that there is a resistor from grid to the ground missing :-). It looks like the grid is "floating". No wonder that you have such problems.

Mark

[redfoxblues]

Its in there I copied the fender layouts so if it was spot on it would be a 250k but I used the next best thing and I think that 4 is the screen grid and 5 is the control grid if that is what you call it grid #1 by the data sheet . I got to thinking that part of my problems could have come from replacing the 6sc7 with a 6sl7. From my research people say they are basically the same but the 6sc7 has connected cathodes and a different pinout. Now that I had thought of it I wonder if a 6sl7 would run exactly the same in a circuit for the 6sc7 without any modification? I found a schematic for an amp called "The Mason" its apparently Canadian and its tube layout is the same as what I have (6sj7,6sl7,2x6v6gt) So when I get the notion im going to scan it over and try to accommodate my circuit to its values and see if that gets me anywhere. Either way I would like to know what is causing my problem for shits and giggles I have always been a curious cat I just hope it doesn't kill me lol. I am thinking from wat I have read and from what is happening that this has something to do with the operating parameters of the phase inverter, hence the changing of the plate resistors making it a little better?

[Enzo]

Can you post a schematic of your circuit, rather than a wiring diagram.

[Chuck H]

As per Enzo and g-one, we need accurate information for it to matter. Mostly accurate information or information that is incomplete is no information at all in this case. If you don't want this to become the classic:

OP) I've got a weird problem with my amp. It's sort of this circuit, but different.

member) It looks like you're missing this or that.

OP) My amp actually has that.

OP) And it's also doing this other thing.

member) Can you post an accurate schematic?

OP) Here's a schematic for an amp that's almost like mine.

member) Do you have this or that in place. You said that your amp uses a different tube than this schematic.

OP) It does, and yes. I just want to find out why my amp is doing this weird thing. Do you think that other thing may be causing it?

???????????????????????????????????????????????????????? ???????????????????????????????????????????????????????? ??????????

We'd rather have an accurate schematic but even some clear photos might help.

[redfoxblues]

I would have to make one or I could give you the two that make up the one amp and tell you where the one ends and the other begins. The first preamp stage is a 6sj7 taken from the 5c1 at the point it goes into the volume pot the schematic would switch to the 5c3 and would leave the pot and tone pot into what is supposed to be the 6sc7 phase inverter of the 5c3 and from there on out its all pretty much 5c3. for the 250k resistors on the phase inverter I used 330k in the beginning as it was all I had. Those values are now different the plate resistors are 180k and the resistor connected to the left 6v6 grid is 390k. I also put 1k grid resistors on pins 5 of the 6v6s. I also changed the bias of the 6sj7 to cathode bias running at -4v and I changed the plate and screen resistors of the 6sj7 to bring the voltages up.

[redfoxblues]

I am sorry if you need me too I will make a schematic I for some reason haven't had the patience to mess with schematics much I have a bachelors degree in biochemistry and im pretty bright but for some reason creating a layout from the schematic eludes me if one is not created already so I guess that's why I have shied away from schems so far but in all reality every amp layout available sheds light on how a schematic could be laid out so maybe im just being a sissy lol.

[redfoxblues]

My bad there fellas I was merely trying to give all the information and relative figures that I had/have at the moment in hopes that it would spark some insight. Next time reply ill have an accurate schematic and some photos. My apologies.

Have a great day
Nate

[MarkusBass]

If you cannot provide the schematic, please provide some photos. It's because your drawings are not consistent with what you write.
If you look at this schematic:

you can see that pin #4 is a grid and pin #5 is cathode (and not control grid as you think). Pin #4 should be connected to the ground with a resistor with value in a range of several Mega Ohms. On your drawing I cannot see it (but you claim that it is there).
Pin #5 is cathode at it should be connected to the ground (even as per the schematic you provided). But on your layout it is connected to the ground with 2.7k resistor. So either your layout is wrong, or you connected the tube incorrectly. A photo could clarify this. Otherwise, no one knows what you are talking about.

Mark

[redfoxblues]

Yea I was wrong on that being a grid here is what I have in my amp I connected everything as per the fender layouts which follow the schematics also by fender I will say since I have been building amps for only 7 months or so that I still truly do not know everything about a tube so my apologies my camera is dead and has to use another with not as good of quality and your right I don't have pin 4 connected to ground anymore since I lifted the 5 meg off of it and cathode biased the 6sj7. The layout I posted is 100% the same as what my amp looks like right now
the amp plays fine short of it losing bias at high volumes and cutting out/loosing umph and a little oscillation which is possibly a layout problem as this developed while messing with things. All I really am worried about it figuring out why it is losing its bias and the phenomenon responsible thanks for looking over my stuff im not trying to be a pain in the ass

[Chuck H]

I don't see a 0V reference resistor on the input stage. Are you using one that isn't indicated in the provided schem?