More Headroom For A Blues Deluxe Reissue

[rich64]

I have this BDRI on the bench that now sounds great with one glaring exception: regardless of the bias setting I can't get much headroom out of it. It has ample output though. I even swapped V1's 12AX7 for a 12AT7. This gave me a little more headroom but not enough. My client is a pedalboard user and therefore counts on it. I also tried a known good speaker only to find that the stock one is fine. Any ideas folks?

[g1]

Loud but distorted? Check both plate resistors on the PI tube, it's not an uncommon fault with these that one of those resistors goes open.

[rich64]

Hey g1, Thanks for the hip tip! Before I pulled the tube board I checked the voltage at the PI plates and came up with 200 to 215 volts. The schemo says to expect around 4 volts! I suppose I'll pull the tube board now to check the plate load resistors. Thank you!

[Dave H]

I can't get much headroom out of it. It has ample output though.

If the PI plate resistors are OK check to see if it will do ample output both clean and distorted even if you have to set the controls to '1' for ample clean output.
I notice from the schematic that the vol and drive controls are set up as variable resistors rather than potentiometers, this spoils the gradual taper and makes them come up too quickly. It's just the way it's designed.

[Enzo]

Easy way yo check the resistors without finding them on the board: measure voltage on pins 1 and 6 of the phase splitter. If a plate resistor is open, the B+ will be missing from that pin.

[J M Fahey]

I can't get much headroom out of it. It has ample output though.

Those are contradicting statements so "to put some numbers into it":

1) please inject 100mV 1kHz into it, set Master Volume to 10, all tone controls (including Presence if present) to 7 , slooowly rise volume while scoping output, stop when you start seeing clipping: what power do you measure?

2) what number is volume set to when it starts clipping?

3) exactly why does your customer complain of lack of headroom? meaning amp starts distorting in an ugly way.
With the flat amp setting I suggested above: at what volume setting does amp start to sound ugly?

a) same as before
b) at a higher volume
c) at a much lower volume setting. In all cases post "the number" at which the knob is pointing.

I even swapped V1's 12AX7 for a 12AT7. This gave me a little more headroom but not enough. My client is a pedalboard user and therefore counts on it. I also tried a known good speaker only to find that the stock one is fine. Any ideas folks?

Yes, the "pedalboard user" bit is a red flag.
Doubly so if coupled to a Blues Deluxe thingie.

4) That amp is officially pushing 40W through a standard 12" speaker ... which means itīs probably 35W clean ... tops.

A perfect combination if you plug your guitar straight into it, you have up to 35W clean which is enough on a mid/small Club setting, and full blast probably does not go beyond 45/50W no matter what, enough to be comfortably heard but killing nobody, drowning nobody, killer creamy power tube distortion.

Now you add a digital simulator ahead of it, which has tons of new sounds , and itīs like fitting 10 pounds of sh*t in a 2 pound bag ..... it wonīt fit, period.

Please do the tests I suggested above, it *might* have a Technical problem after all, which should be solved, but my firm hunch is that the pedalboard output is smashing that preamp input big time, and to boot injecting sounds the amp and speaker combination are not designed to handle.

Not that anything is "broken" , far from it, simply (way) mismatched.

Your Customer needs a loud clean amp for his pedalboard, anything "100/120W" class, SS, 2 x 12" ... tons of amps matching that, from Peavey, Crate, even Fender itself.

Even a humble FM212 will work better ***in that duty*** than a barely 40W Tweed amp.

[glebert]

Maybe try an AT7 in the PI position as well? I found my Hot Rod Deluxe to break up too early, I can't remember if I did V1 and the PI or just V1. In doing research on it everyone basically says "this ain't a clean amp, it is going to break up." It was very sensitive to dynamics, which most distortion is but for whatever reason the Deluxe seemed moreso.

[rich64]

Thank you sooo much for all of your posts! I now have a battle plan for my early morning bench time. Have a great weekend everyone. Rich

[Chuck H]

I oppose all attempts to mitigate gain in search of more headroom. Headroom is a badly misused term WRT guitar amps. Headroom would be how much clean power the amp can make with a given input signal. The knob setting IS ENTIRELY ARBITRARY!!!!!!!!! Trying to get the right amount of gain at a given stock knob setting by using lower gain tubes changes everything else about the amp and is how people with limited knowledge of the real issue tend to approach it. Don't be THAT guy.

It's really all about what the amp will make in watts relative to input. You test for that. If the amp is performing, and I'll bet it is if it's adjusted correctly and not broken, then YOU DO NOT HAVE A HEADROOM PROBLEM!!!!!!!!!!

If the above checks out and the amp is making adequate clean watts then what you really want is a different gain adjustment ratio to "look" more intuitive on the volume knob setting. This is absolutely not the same thing as headroom.

Perhaps look into a replacement volume pot that offers a smaller/slower taper.

[Steve A.]

If the PI plate resistors are OK check to see if it will do ample output both clean and distorted even if you have to set the controls to '1' for ample clean output.

I notice from the schematic that the vol and drive controls are set up as variable resistors rather than potentiometers, this spoils the gradual taper and makes them come up too quickly. It's just the way it's designed.

[ATTACH=CONFIG]49546[/ATTACH]

I guess it was the Hot Rod amps that reconfigured one or both of these controls as 3 terminal potentiometers... So Fender has not changed the BD circuit since 2004? I don't keep up with stuff like that...

I have no idea what the PCB looks like but in my FrankenAmp experiments I have reconfigured pots and tube input grids since they are so easy to trace. Not that I am suggesting it here for a customer's amp...

As for replacing V1 with a 12AT7, I always try a 12AY7, 5751, 12AZ7 or even a 12AU7 first since the bias (or whatever!) is basically the same as a 12AX7, right?

"Pedal pushers" might try plugging into the power amp input...

Steve A.

EDIT Has the bias voltage/current been checked?

[Steve A.]

I oppose all attempts to mitigate gain in search of more headroom. Headroom is a badly misused term WRT guitar amps. Headroom would be how much clean power the amp can make with a given input signal. The knob setting IS ENTIRELY ARBITRARY!!!!!!!!! Trying to get the right amount of gain at a given stock knob setting by using lower gain tubes changes everything else about the amp and is how people with limited knowledge of the real issue tend to approach it. Don't be THAT guy.

I agree with what you said about the knob setting being arbitrary BUT too many modern amps have way too much gain for me so I will try replacing V1 and/or V2 with lower gain members if the 12A_7 family.

Headroom, shredroom... all I care about is TONE. And if a simple tube roll doesn't solve the amp problem then back to GC it goes...

Steve A.

[J M Fahey]

Perhaps look into a replacement volume pot that offers a smaller/slower taper.

Which is a Psychological or at best, Ergonomic solution: " aaaahhhhh!!!! before I could not rise it above 3, now I can rise it to SEVEN!!!!!!! .... I am HAPPY!!!!" while actual power out, and more important, SPL, is **exactly** the same as before.

**REAL** headroom means: "I can play clean by a drummer and be heard, no crunch unless I want to"

Which in practical terms means 30W into a very efficient speaker (Marshall 4 x 12" or 2 of the best Celestions such as in a VOX AC30) or 50W into a normal-good speaker (your average Eminence) or if using SS, 80W into a good Celestion (think Valvestate 80 and such) or 100W into cheapish but still usable speakers (FM212 and most Peavey/Crate/Laney 2 x 12" combos)

Many will rise a hand and say: "hey!! I play very comfortably with 22W 2 x 6V6 1x12" Eminence, never needed more" .... typically such players are 50ish, give/take 10 years, play in a similar age group, usually play "da Blooz" or older style RocknīRoll , have milder drummers, Clubs do not let them play too loud anyway, but most important, always play around the "sweet spot" which means they are already crunching , meaning they are *always* putting out maximum power output (by definition ), and just go somewhat above or below that "sweet spot" by guitar volume adjustment or even finger pressure.
Never *really* clean, at most "Fender clean" which is not the same, just scope it.

Now you plug a Computer into it with 256 sounds, modes, presets, and it simply canīt cope with the clean ones, and will smooth the dirty onbes.

Just try to use some Doom or Death Metal preset
Or any crystal clean one above garage band levels.

[Dave H]

If the above checks out and the amp is making adequate clean watts then what you really want is a different gain adjustment ratio to "look" more intuitive on the volume knob setting. This is absolutely not the same thing as headroom.

Perhaps look into a replacement volume pot that offers a smaller/slower taper.

The term 'Headroom' should be banned. Guitarists and engineers will never agree on what it really means.

II don't think replacing the vol pot will make much difference because it's used as a rheostat not a potentiometer. I think the vol pot is 15% now, even a 10% log pot will only attenuate 12dB (x4) at half rotation not 20dB (x10) as you would expect. The Drive control is the same.

[Chuck H]

I agree with what you said about the knob setting being arbitrary BUT too many modern amps have way too much gain for me so I will try replacing V1 and/or V2 with lower gain members if the 12A_7 family.

Headroom, shredroom... all I care about is TONE. And if a simple tube roll doesn't solve the amp problem then back to GC it goes...

Steve A.

Absolutely! By all means, if it pleases you to have less gain then you can try lowering gain with lower gain tubes

I still oppose doing it in search of more headroom. Assuming one likes everything else about the amp. You can quickly end up in that pickle where "Hey! Now I have the gain I want at five but the amp doesn't rock at ten anymore "

[Chuck H]

I looked into it a little... I can't find significantly detailed specs on the original pot. I did discover that the pot "kit" (available at a popular on line parts vendor) is different for the Blues Deluxe and the Blues Deluxe Reissue.?. They are still bracketed pots. I'm sure the geometry for bracket fit is critical or one has those cumbersome staggered pins. I see this as small consequence. If ONE pot on the board isn't bracketed I don't expect the structural stability would be significantly compromised. I've done this sort of mod many times when I couldn't get an exact replacement pot for such a bracketed pot. It's a little bit of a retrofit, but not usually difficult.

I looked into the Bourns pots but found the Mouser search engine limited for narrowing to PC mount, 15mm, 10% audio. I do like the bourns pots though and I know they offer 10% pots. Worth more research. Mesa also sells pots for their amps and most are 15mm and have their actual taper % indicated. This has been convenient for me at times and is also worth checking out.

You just need to know the value and stock taper of the volume pot that's in there. Then you can look for a fit-able replacement with a smaller taper.