tweaking a '59 RI Bassman LTD

[tubeswell]

While I've got my mate's RI LTD on the bench (having just fixed it with new output tubes and a new (mojo) OT), here I am sitting twiddling on it with my strat thinking "what it is I don't like about the sound?" and it occurs to me that it has a rather ice-picky high end when the presence is dimed. I spoke to him about the ice-pickiness, and he also finds it annoying and is happy for me to experiment a bit and see it it can be tweaked. (Yay!). The PC-board technology in the RI makes changing out any resistor values a real PITA, but apart from gutting the whole amp and doing a 5F6A eyelet board, which I don't really want to be bothered with since its not my amp, I want to think about this carefully about what mods to tweak before diving in.

What I have tried is a 'speed-up' cap tacked in parallel with the 27k NFB series resistor in the RI. The RoT Merlins suggests would mean 74pF is about right (I went with 100pF - and the presence control does sound better for this)

However, to pontificate a bit more, I notice that there are a few differences about the RI circuit compared to a vintage 5F6A. To compare the two (starting from front to back):

The RI http://www.el34world.com/charts/Sche...N_59_REV_E.pdf * has the same input stage as the 5F6A http://www.el34world.com/charts/Sche...SSMAN_5F6A.pdf (more or less) and the same DC-coupled pair driving the tone stack.

(* I couldn't find a schematic for the LTD, but I'm assuming its much the same as the '59 Bassman RI, albeit that the LTD has adjustable bias and a 5AR4 tube rectifier)

The differences between the 5F6A and the RI begin at the tone stack. The TS in RI has a 100k 'slope' resistor (compared with 56k in the 5F6A) and a .1uF cap going to the bass pot input (compared with .022uF in the 5F6A), but the pot values, and the treble and mid caps are the same as the 5F6A. I guess the 56k slope resistor in the 5F6A shifts the mid-scoop point up a couple of hundred Hz and decreases the relative amount of mid scoop available (and is potentially a bit lossier than the 100k slope resistor - not that the latter point should make much difference when being driven by a CF stage). I also guess that the .1uF bass pot cap in the RI interacts to either shunt more bass to ground when the pot is cut, or allows more bass to pass to the next stage when the pot is dimed. The tone controls in the RI circuit give a reasonable range of control, but then again so does the 5F6A tone stack. Horses for courses. A PC-board PITA if I was to change the tone stack to a 5F6A version.

The LTP stages between the RI and the 5F6A are also different, and the RI owes more to the blonde bassman (6G6B) http://www.el34world.com/charts/Sche...SSMAN_6G6B.pdf circuit. The LTP tail resistor is 6k8 in the RI (same as the 6G6B) whereas its 10k in the 5F6A (giving the RI slightly more gain I guess). This is also probably because the RI borrows the presence control from the blonde bassman (but in the RI it is really trebly-peaky when dimed - maybe to do with the OT and speakers in the RI being '5F6A spec'). I note that the LTP plate-to-plate shunt in the 6G6B is 100pF, whereas the RI has retained the 47pF that the 5F6A has. I guess this wouldn't help with the peakyness in the RI, and it may be a simple matter of tacking an extra 100-250pF across the 47pF (that is in the RI) in order to stabilise this somewhat. So that's something relatively easy to try (since an extra 47pF - 100pF can be tacked between Pins 1 and 6 of the LTP socket). (Any suggestions/comments about that?)

Next; the NFB loop in the RI employs a 27k/4k7 voltage divider (similar to the 5F6A), presumably because its riding off the 2000:1 impedance ratio of the RI/5F6A OT. This is probably on a par with the amount of NFB in a 6G6B (but could be why the plate-to-plate shunt in the RI needs upping maybe?). I was wondering whether there could also be a simple opportunity here for 100-200pF shunt caps from the grid-to-ground (as Merlin outlines in Chapter 9 (Fig 9.11) in his first book (to 'slug' the dominant NFB pole as he puts it). It would be a hell of a lot easier to simply put these caps on the output tube socket pins between the grid and the cathode (but the cathode is sitting on top of the 1R bias resistor - does that matter in this regard?).

The output stages of the RI and the 5F6A are virtually identical, but the RI employs 47R grid resistors, which are mounted on the main board (- miles away from the 6L6 grid pins, and exactly what they expect 47R to achieve as grid stoppers for the 6L6s is beyond me). Seems to me that putting 1k5-4k7 grid stoppers at Pin 5 might be an improvement.

Also the 470R screen resistors in the RI are mounted on the main board (also miles away from the screen socket pins). I wonder if it would be better (in terms of making the noise floor of the amp a bit quieter - in the typically high-interference gigging environment - considering stray coupling, oscillations'n'all that) by adding more (~47-100R) g2 resistance in series with each 470R, but mounting these additional resistors on the output tube socket pins? Would (extra) 47R g2 resistors at Pin 4 help with stray RF coupling/stability?

In terms of the power rail, there isn't much difference between the RI and the 5F6A electrically I guess (other than series filter caps with equalising resistors in the RI), except that there is a lot more filter/decoupling for the pre-amp in the RI (possibly to do compensate for something wishy-washy in the grounding design of the PC board?). Would this may make the RI input a bit stiffer/bitier? Again such a pain to have to take the board out to change this.

Sorry about the long-winded post. What do others think? (Also posted this over at ampage, but no takers there yet)

[bluesfendermanblues]

One tweak you might consider is lowering the plate voltage for the V1.

This is done by increasing the 10k (R39 in the reissue 59) in the PSU up to the point where the plate voltage on (pin1 and pin6) on V1 is 150-160V.

You will get a much more responsive tone that way.

Using the stock values on new and old fender tweed amps schematics is of little help in this matter.

In a recent champ 5F1, I tried the stock value in the schematic which is 22k and had to increase this value up to 68k in order to get the voltage down from initial 200V to 160v. The change in to was dramatic in terms of smooth compression mellowness and just plain down right blues tone.

The value of the dropping resistor depends on the preamp tube you use in V1 - they can vary by up to +/-20V IME.

[martin manning]

The differences you note in the tone stack and the PI tail between the RI and the old (1957) Fender schematic floating around have been discussed before. There are photos of '59 5F6-A's that show that the RI values were used in the originals. Presumably Fender chose them with some care, trying to recapture the sound of the best known examples. It's possible that some of what you are hearing is related to the modern speaker and cabinet construction too.

[billyz]

I would changed the grid resistors to 1.5k and put them on the socket where there belong. This will tame some of the ice pick too. And do not discount the speakers in your equations. Are you running the GZ34 or SS diodes in the rectifier circuit? Of course there are too many changes that really need to be made, ie CC resistors, film and foil Caps throughout. another nice one would be to use a Polystyrene 250pf treble cap, smoother/sweeter sounding to my ears.

[tubeswell]

I would changed the grid resistors to 1.5k and put them on the socket where there belong. This will tame some of the ice pick too.

I did just that, and it has taken out some of the really high faint random static at idle.

And do not discount the speakers in your equations.

If it were my amp I'd definitely change the speakers

Are you running the GZ34 or SS diodes in the rectifier circuit?

It has a 5AR4. Gives a B+ around 460, Plates 459, screens 458, and PI plates 279 and 287 with PI cathode at 33.3, so those things are all in the ballpark. (I didn't bother taking the other pre-amp voltages)

Of course there are too many changes that really need to be made, ie CC resistors, film and foil Caps throughout. another nice one would be to use a Polystyrene 250pf treble cap, smoother/sweeter sounding to my ears.

I hear ya. But at this stage I'm just looking for some miniscule tweaks to get the ice-pick out. The 1k5 grid stoppers (mounted on the sockets) and the 100pF speed-up cap (in parallel with the 27k feedback resistor) seem to be just the ticket. Its a very subtle change. But I've only tested it at bedroom levels. If it plays up like that at the next gig, I may yet resort to those 250pF grid-to-ground caps Merlin mentions.

[tubeswell]

One tweak you might consider is lowering the plate voltage for the V1.

This is done by increasing the 10k (R39 in the reissue 59) in the PSU up to the point where the plate voltage on (pin1 and pin6) on V1 is 150-160V.

You will get a much more responsive tone that way.

Using the stock values on new and old fender tweed amps schematics is of little help in this matter.

In a recent champ 5F1, I tried the stock value in the schematic which is 22k and had to increase this value up to 68k in order to get the voltage down from initial 200V to 160v. The change in to was dramatic in terms of smooth compression mellowness and just plain down right blues tone.

The value of the dropping resistor depends on the preamp tube you use in V1 - they can vary by up to +/-20V IME.

Did think about substituting parts on the main board, but at this stage I want to avoid pulling it out, so I'm just going with small changes that are easy to implement

[tubeswell]

The differences you note in the tone stack and the PI tail between the RI and the old (1957) Fender schematic floating around have been discussed before. There are photos of '59 5F6-A's that show that the RI values were used in the originals. Presumably Fender chose them with some care, trying to recapture the sound of the best known examples. It's possible that some of what you are hearing is related to the modern speaker and cabinet construction too.

I think the tone stack works fine. It seems to be just how it interacts with the stock presence control that is annoying. I now seem to have got that subtly tamed (at bedroom levels) with the 100pF speed-up cap in the NFB loop, but I need to test it at gig levels

[billyz]

Keep us posted on your tweaks and the results. These are common amps and we all get to work on them from time to time. .

[Buschman]

I played a reissue for a long time. I also have two originals. I never use the presence dimed on any of them. Try a 5751or a 12ay7 in v1. It is the original tube that was there in the first place. It is a smoother fat tone.

[tubeswell]

Yeah I tried it with a 12AY7 in V1 and it was fatter (as expected), but still had an undesirable ice-pick with the presence dimed (before I added the 100pF speed-up cap to the NFB resistor and the 6L6 grid stoppers at the grid pins).

In the stock configuration, with the presence cut there is no selective treble boost happening and the stock configuration isn't so bad. But why have a presence control if its unusable?

As I have changed it to-date, I think its just about there, but the real test will be the next gig. (The 250pF at each grid-to-ground) is my next back-up plan)

[Gaz]

In TUT2 Kevin O'Connor suggests removing the presence circuit (and hardwiring it on any setting you desire), and replacing it with a Vox style "cut" control (wired in reverse, of course.) He also suggests the addtion of the grid stoppers, which you've already done.

[Structo]

Something wrong about using the terms printed circuit board and tweed amp in the same sentence.

I had thought the Tweed reissues had eyelet boards.
I guess not.

A lot of amps sound unpleasant when you dime the presence pot.

[PaisleyTube]

I've modded a couple of (lTd. and non ltd.) Bassman amplifiers for customers over here, mostly for a SRV-ish style sound just by modifying parts on the pcb. It's slightly more work than a eyeletboard amp but it's certainly possible.

Here are a couple of tips:
- lower the filtering for the 1st preamp-node in the powersupply; 40u is (imo) way overkill, 10-20u is enough and it feels nicer
- adjust the preamp-node-voltage till it's ok
- use a 12AY7 as V1
- wire the 470 ohms G2 stoppers ON the outputtubesockets and also use some (let's say) 1k5 gridstoppers on them
- bias by ear (start low and stop when the high-end gets "fatter")
- the ltd. does have a better cabinet (imo); it's more 3D sounding than the earlier reïssue versions
- the original 5F6A Bassman amps did have a few tonestack-versions like 56k/0.02/0.02, 100k/0.1/0.02, etc. I do prefer 56k & 2x0.02 if using the vintage-specs
- I prefer a MM choke and OT upgrade in such amplifiers

[PaisleyTube]

@structo:

the (more expensive) custom shop tweeds like the '57 Deluxe, '57 Low Power Tweed Twin and the tweed Champ do use eyeletboards, MM iron but cheap caps, average wiring, etc.

The '59 Bassman reï, just like the brwonface and blackface reïssues are pcb-based altough the '64 Vibroverb custom must have an eyeletboard afaik

hope this helps