Page 1 of 2 12 LastLast
Results 1 to 35 of 39

Thread: Adapted 5F2A with EZ80 and EL84

  1. #1
    Senior Member
    Join Date
    Apr 2011
    Posts
    114
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0

    Adapted 5F2A with EZ80 and EL84

    My son and I have just finished building a SE 5 watt amp using a Princeton 5F2A circuit, but we've swapped the 5y3gt rectifier for an EZ80 and the 6V6 for an EL84 (we're in the UK).

    we've compensated by swapping the cathode resistor from 470R to 150R

    The tone at low volume is sweet, but on turning up it breaks up into into a brittle harsh distortion.

    Any suggestions on how to improve the sound please?

    0 Not allowed! Not allowed!

  2. #2
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    5,415
    Thumbs Up/Down
    Received: 406/1
    Given: 325/0
    Schematic?

    0 Not allowed! Not allowed!
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  3. #3
    Senior Member
    Join Date
    Apr 2011
    Posts
    114
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    Quote Originally Posted by The Dude View Post
    Schematic?
    It's just the Princeton 5F2A but with the 5Y3GT replaced with an EZ80 and the 6V6 replaced with and EL84. The 470R cathode resistor is replaced with a 150R

    0 Not allowed! Not allowed!

  4. #4
    Senior Member
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    1,903
    Thumbs Up/Down
    Received: 190/3
    Given: 125/0
    Quote Originally Posted by stephenhartley View Post
    The tone at low volume is sweet, but on turning up it breaks up into into a brittle harsh distortion.

    Any suggestions on how to improve the sound please?
    You probably don't want to hear this but... I used to gig with a 2 x EL84 push pull amp and I liked the sound of it cranked up so I bought a Valve Junior head thinking I could get the same sound at a lower volume at home but It had that harsh distortion. I eventually changed it to 6V6 and it's fine now.

    A couple of less drastic suggestions.

    It will have more negative feedback with an EL84 try doubling the value of the feedback resistor or disconnect it to see (hear) if that helps to tame the harsh breakup. What is the EL84 plate voltage? It may sound smoother at a lower voltage.

    0 Not allowed! Not allowed!

  5. #5
    Old Timer
    Join Date
    May 2006
    Location
    Chill-Ville, VA
    Posts
    2,715
    Thumbs Up/Down
    Received: 264/4
    Given: 555/11
    Paging Chuck, the EL34 Master himself...

    Justin

    1 Not allowed! Not allowed!
    "... If an older Boogie and classic Marshall had a (clearly illegitimate) child and you baked it in an oven set to clown shit crazy." - Chuck H. -
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  6. #6
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,851
    Thumbs Up/Down
    Received: 912/5
    Given: 941/0
    Having worked over a few Champs (similar enough to a Princeton) and having worked with a few el84 builds (but never SE el84)...

    Champs, IMHE, often have harsh distortion. I haven't been able to isolate all the causes, but I have mitigated it many times. The first thing I would suggest would be a shunt filter (conjuctive filter?). A Zobel network across the OT primary. Those hashy spikes don't stand a chance. The tricky part would be choosing a capacitance that doesn't screw up the clean tone. And don't be afraid to make the resistance a little lower than typical for this circuit (which I've read as 1.4x primary impedance). The last Champ I "corrected" used a 2kV 1500p cap straight across the OT.

    The next thing I would mention is that it's easy to over dissipate the screens on el84's. They're not as burly as 6v6's. If the screen is attempting to dissipate power instead of the plate it's both bad for the tube and harsh sounding. Perhaps add a 1k/3W screen grid resistor.

    I have also noticed that the right speaker can help alot. Also, most stock Champ OT's (likely the same OT's sold for the 5F2A?) have really poor LF when you pound on them. The frequency balance favoring HF doesn't help with harshness. Using a bigger OT has helped a lot for me. But these are expensive components so certainly try the above first.

    JM2C

    EDIT: Oh! Also,.. 150 ohms for the cathode resistor may not be hot enough. If the clipped wave form is too wonky it can show a bad spike on the favored swing. What is your plate voltage and what is the voltage on top of the cathode resistor?

    2 Not allowed! Not allowed!
    Last edited by Chuck H; 09-14-2018 at 03:19 PM.
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  7. #7
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    5,569
    Thumbs Up/Down
    Received: 655/15
    Given: 707/0
    Quote Originally Posted by Chuck H View Post
    stock Champ OT's (likely the same OT's sold for the 5F2A?) have really poor LF when you pound on them. The frequency balance favoring HF doesn't help with harshness. Using a bigger OT has helped a lot for me.
    Deafinitely true! If you start with a dinky stock Champ OT, leave some space on the chassis so you can swap in a bigger better SE OT as an upgrade. Also connect your speaker with quarter inch jack & plug so you can opt to drive other speaker cabs with an ordinary cable. You'd be surprised how good a SE Princeton or Champ can sound through bigger speakers, even a 4x12.

    Couple of weeks ago I had a Laney SE combo in with a blown OT. I happened to have a Ceria SE transformer left over from a conversion (SE to PP) project a couple years earlier. Customer & I were both impressed. Him, more so because he got to compare the results before & after the OT swap. I don't know if Ceria sells their OT as an individual item, but I figure any competent SE OT would do better than a thumb-sized Champ OT that rolls off steeply below 400 Hz.

    2 Not allowed! Not allowed!

  8. #8
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,467
    Thumbs Up/Down
    Received: 143/0
    Given: 206/1
    Perhaps the HT is excessive; maybe try dropping it back a bit to see if that helps, eg with an extra filter stage, as per the AC4 https://el34world.com/charts/Schemat...ox/Vox_ac4.pdf

    0 Not allowed! Not allowed!

  9. #9
    Senior Member
    Join Date
    Apr 2011
    Posts
    114
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    Thanks very much

    I've used a Hammond 1760C which has 5000 and 8000 ohm taps - I'm using the 8000 ohm.

    I've got a hefty 60s 10" Goodmans speaker.

    The voltage measurements:

    B+ 237
    plate (pin7) 226
    cathode (pin3) 5
    pin 9 189

    0 Not allowed! Not allowed!

  10. #10
    Senior Member
    Join Date
    Apr 2011
    Posts
    114
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    The PT is a Danbury 1248DT - 190-0-190 @ 80mA

    0 Not allowed! Not allowed!

  11. #11
    Senior Member
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    1,903
    Thumbs Up/Down
    Received: 190/3
    Given: 125/0
    Quote Originally Posted by stephenhartley View Post
    I've used a Hammond 1760C which has 5000 and 8000 ohm taps - I'm using the 8000 ohm.

    I've got a hefty 60s 10" Goodmans speaker.

    The voltage measurements:

    B+ 237
    plate (pin7) 226
    cathode (pin3) 5
    pin 9 189
    Try it on the 5k tap. With a low plate voltage like that it would be closer to centre biased on the 5k tap (like an AC4)

    2 Not allowed! Not allowed!

  12. #12
    Senior Member
    Join Date
    Apr 2011
    Posts
    114
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    Ok - I'll try the 5k tap - should I increase the cathode resistor?

    0 Not allowed! Not allowed!

  13. #13
    Senior Member
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    1,903
    Thumbs Up/Down
    Received: 190/3
    Given: 125/0
    Quote Originally Posted by stephenhartley View Post
    Ok - I'll try the 5k tap - should I increase the cathode resistor?
    No, don't change the cathode resistor.

    1 Not allowed! Not allowed!

  14. #14
    Senior Member
    Join Date
    Apr 2011
    Posts
    114
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    5k tap seemed to make it worse - it cuts into a horrible buzz above 2 on vol now as though something is shorting

    0 Not allowed! Not allowed!

  15. #15
    Senior Member
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    1,903
    Thumbs Up/Down
    Received: 190/3
    Given: 125/0
    How does it sound with the negative feedback resistor connected?

    0 Not allowed! Not allowed!

  16. #16
    Senior Member
    Join Date
    Apr 2011
    Posts
    114
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    I've swapped out the el84 for another and some of the noise has gone - I uncoupled the negative feedback resistor and it was slightly harsher and brighter so I put it back. The same problem remains however that if I turn it above 2, it's breaking up into a horrible brittle distortion. I'll try some of the above suggestions tomorrow - maybe the 5F2A circuit and the 190-0-190 80mA transformer that I'm using just aren't suited to an EL84?

    0 Not allowed! Not allowed!

  17. #17
    Lest We Forget g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    10,415
    Thumbs Up/Down
    Received: 582/21
    Given: 1,286/5
    Your heater voltage is holding up? I'm only asking because this rectifier runs on 6.3V heater.

    0 Not allowed! Not allowed!
    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

  18. #18
    Senior Member
    Join Date
    Apr 2011
    Posts
    114
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    The heater circuit (6.3v) runs from the pilot light to the rectifier to the el84 to the ECC83. It doesn't have a centre tap - I've earthed it to the cathode resistor earth as recommended on here by one of the American experts. I don't think the heaters are the problem.

    0 Not allowed! Not allowed!

  19. #19
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,851
    Thumbs Up/Down
    Received: 912/5
    Given: 941/0
    You're only biased at 7W idling (total dissipation***) with 221 working volts on the plate. You should be entirely safe reducing the cathode resistance to get closer to center bias. Try a 100R cathode. Maybe even lower. Ideal would be about 12w or 13W at idle. Maybe even 14W because the bias does get colder as the tube conducts signal. There are even market examples of amps running the el84/s at that level with higher plate voltage. ***One thing to consider is that the 11W max plate dissipation doesn't include the screen dissipation, which is part of the total tube dissipation as measured at the cathode resistor. Right now you're at 7W. That means something more like 5.5W or at most 6W of plate dissipation for a class A amp that should be at 100% plate dissipation for center bias.

    I'd try a 100r cathode resistor and remeasure. You may even need to go lower.

    1 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  20. #20
    Senior Member
    Join Date
    Apr 2011
    Posts
    114
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    Thanks - I'll give it a try. I'll try the 5000 ohm tap again now that I've replaced the faulty el84.

    0 Not allowed! Not allowed!

  21. #21
    Senior Member
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    1,903
    Thumbs Up/Down
    Received: 190/3
    Given: 125/0
    Quote Originally Posted by Chuck H View Post
    Right now you're at 7W. That means something more like 5.5W or at most 6W of plate dissipation for a class A amp that should be at 100% plate dissipation for center bias.

    I'd try a 100r cathode resistor and remeasure. You may even need to go lower.
    Chuck, With the plate voltage and OT impedance it has I don't think it can be centre biased at 100% plate dissipation. It's close to centre bias now at 7W, 30mA on the 5k tap. It would need a lower impedance OT to be centre biased at 100%. Having said that it's a guitar amp, it doesn't have to be centre biased. If it sounds better with a lower cathode resistor then go for it.

    SE class A amps are usually designed to run centre biased at the rated Pd max of the tube for maximum power output but the don't have to be, they could be designed to be centre biased at a lower dissipation for longer tube life.

    3 Not allowed! Not allowed!

  22. #22
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,851
    Thumbs Up/Down
    Received: 912/5
    Given: 941/0
    Quote Originally Posted by Dave H View Post
    Chuck, With the plate voltage and OT impedance it has I don't think it can be centre biased at 100% plate dissipation. It's close to centre bias now at 7W, 30mA on the 5k tap. It would need a lower impedance OT to be centre biased at 100%. Having said that it's a guitar amp, it doesn't have to be centre biased. If it sounds better with a lower cathode resistor then go for it.

    SE class A amps are usually designed to run centre biased at the rated Pd max of the tube for maximum power output but the don't have to be, they could be designed to be centre biased at a lower dissipation for longer tube life.
    Thanks Dave. I've never been able to wrap my head around load lines. I'm working from bench experience and example circuits. In which case 150R for the cathode with 220-ish working volts on the plate would be cold for a guitar amp. I'm also considering that the bias will cool considerably when the amp is clipping hard. On the bench what I've seen in that case is a higher spike on the positive swing. When biased so that the clipped wave form is more symmetrical that spike is reduced. YMMV.

    0 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  23. #23
    Senior Member
    Join Date
    Apr 2011
    Posts
    114
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    Thanks for the expert advice everyone. It's much appreciated.

    I'll get back on it next week when I have time, and report back - one change at a time to see what works - I'll measure the voltages at each stage:
    1. OT back to the 5k tap from the 8k tap
    2. lower the cathode resistor
    3. Double up the feedback loop resistor
    4. screen grid resistor
    5. dropping HT with an extra filter stage (like AC4) - that one's a bit beyond my novice scope

    0 Not allowed! Not allowed!

  24. #24
    Old Timer Tom Phillips's Avatar
    Join Date
    Jun 2006
    Location
    California, USA
    Posts
    3,554
    Thumbs Up/Down
    Received: 128/0
    Given: 90/1
    Quote Originally Posted by stephenhartley View Post
    ...5. dropping HT with an extra filter stage (like AC4) - that one's a bit beyond my novice scope
    That change will also lower the hum that exists in many small single ended guitar amps. This is because the extra filter stage will reduce the ripple in the output stage plate supply. For a given level of ripple a single ended power amp will hum more than a Push Pull output stage because the single ended amp does not benefit from the hum cancelling that is inherent in the PP power amp circuit.

    0 Not allowed! Not allowed!

  25. #25
    Senior Member
    Join Date
    Apr 2011
    Posts
    114
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    it does have quite a loud buzz which disappears when I plug the guitar in. I've spent a few hours today experimenting with different cathode and grid resistors and no matter what I try, I can't seem to get the dissipation wattage above 7. The battery on my multimeter went so it's time to give it a rest and have a fresh start.

    I have two 60s SE EL84 amps - a Selmer little giant and a Broadway, both with awesome sound and I wanted to replicate that but keep the early Fender style build that I've used on my 20w & 30w builds - hence the rational of trying to adapt the 5F2A to an EZ80 and EL84.

    0 Not allowed! Not allowed!

  26. #26
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,851
    Thumbs Up/Down
    Received: 912/5
    Given: 941/0
    Quote Originally Posted by stephenhartley View Post
    it does have quite a loud buzz which disappears when I plug the guitar in.
    That's almost surely a wiring error.

    Quote Originally Posted by stephenhartley View Post
    I've spent a few hours today experimenting with different cathode and grid resistors and no matter what I try, I can't seem to get the dissipation wattage above 7.
    I anticipated this could happen since cathode bias is self adjusting. I still think you'll be better with the 100r cathode though because it should reduce bias shifting cooler when the amp is clipping. You duplicated the 150r value from the Selmer design, but notice that the Selmer's plate is some 60V higher than this build.

    Quote Originally Posted by stephenhartley View Post
    I wanted to replicate that but keep the early Fender style build that I've used on my 20w & 30w builds - hence the rational of trying to adapt the 5F2A to an EZ80 and EL84.
    There's nothing inherently wrong with your tube substitution. The el84 will be more easily driven to clipping by the preamp circuit than a 6v6 would. No great shakes. If it bothers you, you can always reduce drive voltage. Your plate voltage is quite low compared to similar amps. This would reduce output some relative to what you're use to perhaps. I think the combination of the lower output and the el84 clipping at lower drive voltage than a 6v6 may be creating a sort of illusion of low headroom. The amp may actually be making all the watts it can before clipping even though that starts at vol-2. It may also partly explain the harsh tone. Perhaps there's just too much drive voltage on the el84 grid. You can really only clip a single tube so much before reaching a point of diminishing returns where bad things start to happen. I've had this problem in two single ended amps I built. In both cases the amps sounded harsh and started to roast the screen grid resistor. Limiting drive to the grid (and reducing my expectations for clipping level) was the solution.

    1 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  27. #27
    Old Timer Tom Phillips's Avatar
    Join Date
    Jun 2006
    Location
    California, USA
    Posts
    3,554
    Thumbs Up/Down
    Received: 128/0
    Given: 90/1
    Quote Originally Posted by stephenhartley View Post
    it does have quite a loud buzz which disappears when I plug the guitar in...
    Quote Originally Posted by Chuck H View Post
    That's almost surely a wiring error...
    I agree and my guess is that the sorting bar on the input jack is not grounded.

    0 Not allowed! Not allowed!

  28. #28
    Senior Member
    Join Date
    Apr 2011
    Posts
    114
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    Quote Originally Posted by Tom Phillips View Post
    I agree and my guess is that the sorting bar on the input jack is not grounded.
    Both input sockets are grounded with their own wires to a dedicated ground along with the cathode resistor from the first half of the pre-amp tube. The pots are grounded separately then the filter caps have their own ground. The +ve side of the heater circuit (no centre tap) is grounded to the - ve of the EL84 cathode resistor which too has its own ground. There's obviously a problem with my grounding scheme - it's just finding it.

    'Perhaps there's just too much drive voltage on the el84 grid'

    The grid voltage has been measuring ~ 170v - how do I alter it?

    0 Not allowed! Not allowed!

  29. #29
    Senior Member
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    1,903
    Thumbs Up/Down
    Received: 190/3
    Given: 125/0
    Quote Originally Posted by stephenhartley View Post
    'Perhaps there's just too much drive voltage on the el84 grid'

    The grid voltage has been measuring ~ 170v - how do I alter it?
    Is that 170V DC on the screen grid pin 9?
    It's the signal voltage on the control grid pin 2 that's too high.
    If the amp vol is below 2 the tone control won't work very well which could be part of the problem. As a test try turning the guitar's vol pot down so the amp's vol pot can be set to 5 and still be clean then adjust the amp's tone pot as required. The amp tone pot will work better with its vol pot set to 5. If that sounds OK the gain of the amp could be reduced to make its vol/tone controls have a more useful range but it will never have much clean volume. I'd guess it's only about 2W because of the low B+

    1 Not allowed! Not allowed!
    Last edited by Dave H; 09-17-2018 at 12:05 PM.

  30. #30
    Old Timer Tom Phillips's Avatar
    Join Date
    Jun 2006
    Location
    California, USA
    Posts
    3,554
    Thumbs Up/Down
    Received: 128/0
    Given: 90/1
    Quote Originally Posted by stephenhartley View Post
    Both input sockets are grounded with their own wires to a dedicated ground along with the cathode resistor from the first half of the pre-amp tube...
    Understood. However, that is a different issue than what I was discussion in post #27. If the buzz you mentioned stops when you plug in the guitar then I don't believe it is caused by the grounding topology that you just described. Are you using shorting jacks for your inputs? If so then the tip shunt switch contact should be grounded so that the input is shorted to ground when you unplug the guitar cord. I'm still wondering what type of input jacks you are using and how they are wired.

    0 Not allowed! Not allowed!

  31. #31
    Senior Member
    Join Date
    Apr 2011
    Posts
    114
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    Quote Originally Posted by Tom Phillips View Post
    Understood. However, that is a different issue than what I was discussion in post #27. If the buzz you mentioned stops when you plug in the guitar then I don't believe it is caused by the grounding topology that you just described. Are you using shorting jacks for your inputs? If so then the tip shunt switch contact should be grounded so that the input is shorted to ground when you unplug the guitar cord. I'm still wondering what type of input jacks you are using and how they are wired.
    Thanks - I'm using Switchcraft switched jacks. I've just copied the Fender wiring on the layout, but I'll check it again.

    I'm beginning to think that I should just change the PT - I struggled to find one in the uk. The Hammond one is a huge hefty thing that's big enough for a 5E3 and has a 5v tap which I don't need for an EZ80.

    0 Not allowed! Not allowed!

  32. #32
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,851
    Thumbs Up/Down
    Received: 912/5
    Given: 941/0
    Here's a copy of the 5f2a schematic altered to show the changes I would suggest in red. I know it was suggested that the nfb be increased. I disagree because the harsh tone in these little SE amps is usually symptomatic of other issues and (IMHO) el84's just sound better with a little LESS nfb. In light of your consideration of changing the PT it should be noted that the change to the HV rail resistor feeding the preamp tube was considered with the PT you're using now.

    EDIT: It seems the forum will not let me attach files at this time. Will explore other avenues.

    EDIT 2: Ok... No way to attach images from your PC anymore. It was already hinky, but now it's actually gone. Sorry.

    0 Not allowed! Not allowed!
    Last edited by Chuck H; 09-18-2018 at 06:23 AM.
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  33. #33
    Senior Member
    Join Date
    Apr 2011
    Posts
    114
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    Thank you so much for your effort Chuck.

    I abandoned ship last night and swapped the PT for a Hammond 291AEX using the 550 tap (it reads 588).

    I won't have time to work on it for a couple of days.

    I would love to see the schematic - I'll see if I can message you.

    Tried to upload some pics but doesn't seem to work.

    0 Not allowed! Not allowed!

  34. #34
    Senior Member
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    1,903
    Thumbs Up/Down
    Received: 190/3
    Given: 125/0
    Quote Originally Posted by Chuck H View Post
    Here's a copy of the 5f2a schematic altered to show the changes I would suggest in red. I know it was suggested that the nfb be increased. I disagree because the harsh tone in these little SE amps is usually symptomatic of other issues and (IMHO) el84's just sound better with a little LESS nfb.
    The suggestion was to increase (double) the feedback resistor value to decrease the nfb

    Click image for larger version. 

Name:	princeton_5f2-a_schem.gif 
Views:	31 
Size:	26.5 KB 
ID:	50457

    If you can see the above attachment pm the marked up one to me and I'll upload it.

    1 Not allowed! Not allowed!

  35. #35
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,851
    Thumbs Up/Down
    Received: 912/5
    Given: 941/0
    Ah! Ok. Guess I read through that too fast... Good suggestion

    0 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Modifying 5F2A circuit for EZ80 and EL84
    By stephenhartley in forum Mods & Tweaks
    Replies: 5
    Last Post: 08-06-2018, 06:02 PM
  2. Replies: 49
    Last Post: 10-08-2017, 03:45 AM
  3. EZ80 Rectifier & Circuit Questions
    By mbratch in forum Theory & Design
    Replies: 9
    Last Post: 03-13-2014, 02:59 AM
  4. Crate VC-3112 Combo 4 EL84 To 2 EL84 conversion?
    By Lostfollicles in forum Guitar Amps
    Replies: 5
    Last Post: 10-29-2013, 01:31 PM
  5. Rectifiers: EZ81 in place of an EZ80 - is it possible?
    By slidincharlie (Carlo P) in forum Mods & Tweaks
    Replies: 7
    Last Post: 08-11-2010, 05:49 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •