Preamp Bias Mods 5e5a

gpw5150

Hi All,

New to this forum, but need some advice from amp techs/builders:

Fender '57 5e5a re-issue eyelet board / handwired.

I have modified the preamp to be more like the more basic 5d5: - 

V1 - Was - 820r 22uF - 12AY7
V1 - now - 470r + 22uf - 5751
V1 - Intended - 470r + 25uF - 5751

V2 - was - 1k5r + 22uF - 12AY7
V2 - now - 470r + 22uF - 12 AY7
V2 - intended - 820r + 25uF - 12AY7 - slightly colder bias

Will the intended changes above make any audible difference?
Do I run the risk of running the preamp tubes too hot?
Given the components have ~10% tolerance - are the above changes worth it?

**I have removed the local feedback loop in the preamp - removed 4.7m and 220k completely

PI was - 12Ax7
PI now - 5751
(PI appears to be fixed bias)

As I am reducing gain in the the 5881 tubes, do I run into a bias issue?  The power amp is converted to cathode bias and running 470r 5 watt resistor and a 25uF 50v cap (measured at ~30v - hence the larger v cap)


The amp absolutely roars and sounds phenomenal at about 4-5 on the volume - huge overdrive and usable breakup.  I have modified based on ear as opposed to maths, but have done the maths!  However, I lack the experience to know if things are gonna go bang.

No smoke is escaping and the amp does not appear to overheat.

RiftAmps

gpw5150 said:

Hi All,

New to this forum, but need some advice from amp techs/builders:

Fender '57 5e5a re-issue eyelet board / handwired.

I have modified the preamp to be more like the more basic 5d5: - 

V1 - Was - 820r 22uF - 12AY7
V1 - now - 470r + 22uf - 5751
V1 - Intended - 470r + 25uF - 5751

V2 - was - 1k5r + 22uF - 12AY7
V2 - now - 470r + 22uF - 12 AY7
V2 - intended - 820r + 25uF - 12AY7 - slightly colder bias

Will the intended changes above make any audible difference?
Do I run the risk of running the preamp tubes too hot?
Given the components have ~10% tolerance - are the above changes worth it?

Ohms and Joules laws will give you all of the answers that you need. Take measurements, do your sums, draw grid-curves, check datasheets, work it all out.

PI was - 12Ax7
PI now - 5751
(PI appears to be fixed bias)

As I am reducing gain in the the 5881 tubes, do I run into a bias issue?  The power amp is converted to cathode bias and running 470r 5 watt resistor and a 25uF 50v cap (measured at ~30v - hence the larger v cap)

The reduced voltage swing from the PI won't have any effect on the OPV bias, that's a separate issue.

If the circuit is running within its own limits and you like the resulting sound, then run with it.

gpw5150

Thanks Chris, i was worried that the reduced gain into the poweramp section may affect something or having a hot bias with imbalance in the pre-amp may bork something and cause oscillation in the PI network.

will do the math….reluctantly.

**running 12ax7’s in v1 and v2 does cause oscillation at high volume, but I am and will not be running the highest gain tubes and tbh, I kinda expected it given the lack of local feedback loop in the preamp.

Electrically, nothing changes though in the poweramp, so all good!

ecc83

Yes, it is impossible for anyone to comment on the "safety" of those bias choices without the values of the various HTs involved. Take great care of course but measure anode supply and anode voltages then cathode voltages and do your Ohm's Laws! In the case of the 5881s you also need the G2 volts and the feed resistor value to ensure G2 current is within 'book' value.

IMHO it is rarely worth deviating from data sheet cathode resistor values for preamp valves.

Dave.

gpw5150

My confusion is that if you look at the history of the Fender Tweed Pro amps - as they move from an octal preamp to 12__7 preamp tubes, the bias seems to evolve from 470r shared cathode between v1 and v2, to 470 on v1 and 1k5 on v2 to the final version of 820 v1 and 1k5 v2 all on a 25uF cap.

There is a definite heritage and evolution to the circuit, though the 5d5 seems to be the circuit of choice for vintage overdriven tweed sounds.  

5e5 and previous models only have 2 vols and a tone.  5e5a has Treble, Bass and Presence.

Hence the experiment.  Bridging the channels is awesome - it is like a fire breathing monster!

*I also removed the bright cap as this seems to stiffen the sound and removes some ‘squelch’ from the drive sound.