Preparing for my tweed Bassman 5F6-A build

[osing]

Howdy,


I am starting to plan my tweed Bassman (5F6-A) build and am working on the layout as I will be adding some mods (including switchable tube/SS rectifier, alternate channel voicing, adjustable bias, etc.). The spec for my PT is 380-0-380, and I am considering at least planning for the possibility of lowering B+. In thinking about it, I might be neat to add this as a switchable option. From what I have read here and elsewhere, a simple way to lower B+ is the use of several zener diodes in a series string. Since I do not have a separate bias winding, my understanding is that it is better to insert it between the rectifier and first filter caps versus between ground and the CT.


Some questions you might be able to shed light on:


1) As I will have a switchable rectifier via my standby switch (basically set up as TUBE-STANDBY-SS), I would place the zeners after the standby switch so they would work in either tube or SS mode. Any issues with this as far as what happens when you take the amp off standby? I should perhaps mention that I plan on having a 100k 3W resistor across the standby switch which will always allow some current to flow to the filter caps even in standby mode.


2) Do I need to be concerned about any specs other than the voltage drop and power handling of the zener diodes, especially considering that they will be at B+ level voltages?


3) I seem to have read differing opinions on using heat sinks. If I go with several 10V 5W diodes in series (qty to be determined later), are heat sinks necessary truly necessary if the dissipation is only on the order of a couple of Watts for each diode?


4) Has anyone experienced added noise using zener diodes?


5) If one did place the zener diodes between CT and ground, could you use a DPDT switch to have a removable pot (essentially a second bias pot) allowing you to first set the bias with the zeners in circuit, and then tweak the bias further using the second added pot with the zeners not in the circuit? Or, are there other issues with this approach?


As an FYI, I do plan on building the amp as stock as possible initially as I want to ensure I get it working and also have a reference before adding the various mods, but I am also trying to think ahead about things I might want to add or modify as they affect the layout somewhat.


Appreciate any insights anyone has!

[sluckey]

Since you're only 'starting to plan' wouldn't it make more sense to plan to use a 325-0-325 PT with a bias tap? Sure would simplify things.

[osing]

Well, I already have the PT and probably could have done some better research before acquiring it :-)

[sluckey]

I kinda figured you already had it. 380-0-380 will give you 537VDC unloaded. That is significantly higher than what you probably want. The current capacity of that PT will determine how much that B+ drops when loaded. Higher current capacity means less drop. Your filter caps will need to withstand 537V.

Did you acquire this PT specifically for this project?

[Jack_Hester]

As you are staying with the fixed Bias, you may consider a mod to the Bias circuit to make it adjustable. 

The Gibson Thor Bias circuit is closer to the '59 Bassman, and the attached drawing shows the mod that I made on mine.  Ignore the voltages, as you will have to tweak your dropping resistor to get the total in range.  Set the Level pot to center and play with the dropping resistor value, until you are close to where you want to be on the Bias voltage.  Fine tune it with the level, when the tubes are installed. 

Also, I've included a drawing of a mod that I made to my Guild Thunderstar Bass.  It already had the Bias Balance pots, so I added the Bias Level adjust to that circuit.  This is where a breadboard comes in handy to test these circuits before implementing. 

But as you are in the design to build stage, you can make it what you want. 

Jack

[osing]

Sluckey - I did buy this Mercury Magnetics PT for this project. I had actually called them and asked which PT they typically recommend for the Tweed Bassman 5F6-A as they had quite a few options and I was having a hard time figuring out what to get, and he suggested the one I got (FTBP-58). I did ask about the voltages being quite a bit higher than what shows on the 5F6-A schematic, and he said that given the +/- 20% variances stated on the schematic, it would be within range (albeit it at the very top). He said they felt that this PT sounds the best on these amps. This is actually the cheapest PT listed on their site for the Bassman, so I have no reason to doubt that the guy I spoke with does like this one the best (i.e. he did not try to sell me on the more expensive ones). I assume I will get more headroom with this PT, which is why I am thinking it would be interesting to have way to switch to lower B+ using the zener diodes for different sounds. I have never played a Bassman so that does not help....


That being said, I have been wondering about the filter caps. I bought the 22uF / 500V caps from Doug already with the intent of using them as per the schematic (i.e. the first two in parallel for 40uF / 500V), but I guess I should consider getting a couple of 70uF or 100uF 350V caps and stack them to be safe on the voltage rating. Is it a bad idea to wire it up and see where the voltages land and then decide if I need to change the filter caps? I assume they would not blow immediately if indeed they do get hit with 530V while doing measurements, or is that a bad assumption do you think? I am going to use a cap can so changing them out will be relatively easy.



As far as using the zener diodes, any thoughts on whether it is ok to put them after the standby switch or whether it would work on the CT with an additional bias put switched in and out?


Jack - in the drawing of the Guild bias supply with the balanced bias pots, would you technically need the third pot, or could you not achieve the same thing with the two balance pots as is? Is it essentially so that you can first balance the bias on the two tubes, then bring the bias up or down for both using the third (i.e. as opposed to tweaking one at a time)?

[Jack_Hester]

In the drawing of the Guild bias supply with the balanced bias pots, would you technically need the third pot, or could you not achieve the same thing with the two balance pots as is?

Is it essentially so that you can first balance the bias on the two tubes, then bring the bias up or down for both using the third (i.e. as opposed to tweaking one at a time)?

The Guild uses 3K pots, making for very little range of adjustment.  I suppose that back when the amp was new, and tubes were held to some close specification, you would pop a set in (or replace just one) and balance them.  And, tweaking both to get the overall Bias in range. 

But, age of components probably plays the biggest role in imbalance.  With the Level pot, you bring them both in range, whether you balance before or after. 

I suppose that you could use a bit larger value of Balance pots, and eliminate the Level.  Many options besides the ones that I gave.  These drawings make for a good starting point in the thinking. 

Jack

[plexi50]

A 380-0-380 PT puts much higher voltages throughout the entire amp. To me the magic of the vintage Tweed tone is due to the lower plate voltage on the power tubes and preamp. The Reissue Bassmans use a 380-0-380 but they don't really sound like an original tweed 5F6A either.

All that being said, i know that Fender used higher voltage PT's in some of the vintage 5F6A Bassmans.
That's one reason why they all sound different. I would go with the 325-0-325 PT.
The higher the voltage,the louder the amp. To me at the sacrifice of that shimmering Tweed tone.

[osing]

I think I will try my PT and then if I don't like it, I'll buy a lower voltage one to compare. It seems like the consensus is the lower voltage PT will ultimately sound closer to an original Bassman, so that probably is the route to ultimately take. For now, using my higher voltage PT, since my filter caps are all 500V, I guess I will need to deviate slightly from the 5F6-A schematic. For the first filter stage, I will probably get a couple of 100uF 350V caps and use the totem pole configuration with 220k balancing resistors (effective capacitance would be 50uF vs. 40uF in the schematic). For the second filtering stage (right after the choke), since the voltage is presumably close to the first stage, can I just use a totem pole configuration there as well, perhaps using 47uF caps for an effective capacitance of 23.5uF (schematic calls for 20uF)? I don't think I have ever seen two sets of "totem poles", and I am just curious if there is a reason not to go that route.


On a side note, if I do have a resistor across the standby switch, thereby allowing the filter caps to charge slowly while in standby (all filter caps are after the standby switch), would that in any way sufficiently eliminate the large surge that typically hits the filter caps when the amp is moved off standby? If so, would the original 500V filter caps perhaps be sufficient even with this high voltage PT (380-0-380)?

[MFowler]

Some good info here: https://robrobinette.com/5F6A_Modifications.htm

[osing]

That is indeed a great site that Rob has put together and I have been reading about the mods he explains and plan on trying out several of them!

[osing]

Looks like I found an answer to my dual totem pole question....the '64 Vibroverb with Diaz mods has a pair of 220uFs and a pair of 47uFs.

[Willabe]

I don't think I have ever seen two sets of "totem poles", and I am just curious if there is a reason not to go that route.

Yes there can be.

2 caps take up more space than 1 cap.

And 2 caps cost twice as much as 1 cap plus the 2 balancing R's. So your paying for 4x the capacitance than you end up with, but you do get twice the voltage rating.

Traynor and was it Sound City or HiWatt had some amps power supplies set up with stacked caps for more than 1 B+ node.

You can do every B+ node with stacked caps.

[sluckey]

Several of the high power Marshalls also have two sets of stacked caps.