Minor issues with a new build

[Howlin' Mad Mac]

Just finished a 5F1 build with the following modifications: 12AX7 pin 3 bypass cap 2.2uf, 12AX7 pin 8 820R (cold bias) + 22uf bypass cap, 6V6 pin 8 100uf bypass cap, no nfb, supply filter 40uf/20uf/20uf

Results: a bit bright (as expected), too much sag, very weak output @ 50% volume & below, a sort of raspberry sound @ 100% volume on low E string with strong pick attack... any suggestions? I'd like to decrease the sag, darken the tone just a smidge & definitely get rid of the raspberries... oh, & if it's not too much to ask a bit more oomph in general.

[Howlin' Mad Mac]

Hmmmm... no thoughts anybody? Well here are some of the ideas popping into my head, maybe some kind soul can help point me in the right direction then. Idea No. 1 - add 2nd 2.2uf bypass cap in parallel w/ 1st, should double capacitance but still retain brighter tone from stock 25uf cap? Idea No. 2 - swap out 820R (cold bias) for 1K5 (normal bias), leave 22uf cap in place? Idea No. 3 - decrease value of 6V6 cathode cap by adding 2nd 100uf in series, I'm thinking this should allow more low freq. therefore "tighten up the bass"? Idea No. 4 - Reservoir cap @ 40uf is max. for 22772 transformer (but US made, should be built to over spec therefore able to handle it) & Sovtek 5Y3 tube??? (well what can one say... it's Russian, but hey they try now (just kidding, poor attempt @ humor)), I've added a pair of 1N4007 diodes in series between transformer & 5Y3 pins 4 & 6purely as a precaution in case of tube failure as I've done on most of my builds... anyway perhaps a resistor in parallel w/ reservoir (value?/pwr rating? (I've 1K, 8K2, 10K, 22K/3W & 150R/5W + oodles of 1/2W values on hand)) to keep the pump primed? Idea No. 5 - perhaps try to coax a little more gain out of the beast by increasing the 12AX7 plate resistors? They're both 100K now... How high is ok to go? I've seen Vox schematics using 220K. Idea No. 6 - Perhaps a 470pf HF filter cap across 12AX7 plate resistors? Idea No. 7 - add nfb? Help!

[Chuck H]

With an amp as basic as a 5f1 I think much of the "oomph" and overall EQ comes from the speakers and transformers. Minor circuit tweaks seem to get washed away when you crank it up.

You shouldn't be getting too much sag if the amp is biased correctly. Since single ended amps should be drawing as much current at idle as full output whats to sag? In truth there is often a little sag as the amp begins to clip but it shouldn't be excessive. What is your bias reading?

Most of the brightness when overdriving is due to harmonics and overtones from clipping. You can try a "bleeder" of some kind but the results will probably affect the clean signal more than the clipped.

What transformers and speaker did you use?

[Howlin' Mad Mac]

Bias: 12AX7 1st cathode is 1K5, 2nd is 820R & 6V6 is 470R/5W, Transformers: Power is 22772 from Chicago Magnetics & Output is a Champ replacement from Hammond, As for speakers I tried it with an Egnater 112 cabinet (8 ohms) & a Peavey Penta 412 (16 ohms) both are mismatched to Champ's 4 ohm output but I've used lots of Champs with 8 & 16 ohm cabs & have never had a problem but I wouldn't do this with higher output amps as they don't handle the mismatch as well. Cab I'm building for this amp will have 2 1957 vintage Jensen alnico 10's pulled from an old Hammond chord organ & in series will have the proper impedance.

[loudthud]

Running the 5F1 with a high speaker impedance means excessive screen current. That can cause B+ at the screen node to sag excessively which is fed downstream to the preamp. Because of the big filter caps, the B+ will be slow to recover. A large reservoir cap also increases transformer and rectifier losses which at some point will increase sag. Try a 20uF in the first position. Also try reducing the 10K to something in the 5K to 2K range.

You want to run the Jensen 10s in parallel for 4 ohms.

[Chuck H]

+++ to Mr. thud.
What I was asking about was actual voltage readings. What is your cathode resistor value and what is the voltage on top of it? What is your plate voltage?

[Howlin' Mad Mac]

Ah, now that makes sense & easy enough to implement as my filter cap is a JJ 40/20/20/20 so I just need to swap the reservoir from the 40 to the unused 20. As for the resistor I'm currently using an 8K2/3W but with a 1K/1W between the tap & the screen. Perhaps if I just lose the 1K/1W & add a 2nd 8K2/3W in parallel with the 1st? As for the speaker wiring... Doh! What was I thinking... yea I would need to wire them in parallel or I'd be right back @ 16 ohms, wouldn't I? Oh, & sorry Chuck, I know I should've posted voltages but I haven't had the time to throw a meter on it yet. Will post once I've had the chance. So anyway that should fix the sag... do you think that's where the "raspberry" sound is also coming from? I've been thinking of adding a fixed tone stack (will post schematic once I've finished) between the 1st coupling cap & the volume pot to shape up the sound a bit more. Good or bad idea?

[Howlin' Mad Mac]

Another day, new dilemmas

Ok, new issues... posting schematic & pics this time so ya'll can look it over & maybe that'll help & will dig out my trusty ol' meter this weekend so ya'll can see what my voltages are. Anyway as previously stated it's a 5F1 at heart with minor changes... the most problematic so far seems to be the bigger filter caps. At present amp follows schematic pretty close except without tonestack shown & as Loudthud brought up the big reservoir wasn't recharging quick enough so swapped out 8K2/3W & 22K/3W for 1K/3W & 10K/3W respectively, also changed 6V6 cathode cap from 100uf to typical 22uf, increased 12AX7b cathode resistor to 1K5 from 820R & 12AX7a cathode is now looking at 2K2 in parallel with .68uf. No more sag & lots of oomph & still almost dead quiet at rest even with my Tele however at just beyond 1/2 way on the ol' volume knob starts a low-midrange howl & the pitch varies as I continue to turn the knob then just before pot is completely open howl goes quiet then really goes apesh*t. Strangely guitar's tone thru it all ain't too bad 'cept @ 9-10 on the dial sounds like the clipped waves are clipping. Is this due to no nfb or will the tonestack calm it down. Hopefully ya'll gots some good ideas for me 'cause this one's real close to finished.

6V6 SE.pdf
6V6 SE Power Supply.pdf

[loudthud]

You might want to try tweeking on the 500pF in the tone stack. Seemingly small changes will tune the mid frequency turnover point. Values like 390, 330, 270 as well as intermediate values from parallel combinations might be more to your liking.

[Chuck H]

What's up with that filament circuit??? You have one resistor in series and another as a shunt. Is this to reduce voltage?

If the amp goes that whacky I don't think the mid turnover frequency is going change things enough to stabilize the amp. You may have a lead dress or layout problem. Can you give us a gut shot?

[Howlin' Mad Mac]

Cool. You usually see something in the 250-290pf range but then I see a lot of people tweaking these to 470-500pf so wasn't sure which way to go. Do you think a tone stack (& the extra resistance it'll provide) will eliminate the feedback howl this thing seems to have or do I need to add the nfb resistor to get rid of that? Another possibility could be a need for a large power resistor (100R/10W) between the rectifier & the reservoir to dampen it just a bit? At any rate I did take some pix... maybe somebody will see something I overlooked.

[tubeswell]

That is a heavily loaded V1 stage for little apparent benefit (because you can't adjust the tone circuit). You might get better results with just a plain old 5F2A tone stack (or no tone stack at all?), and a smaller V1 coupling cap - 1n to 4n7)

[Howlin' Mad Mac]

Oops! Those resistors are drawn wrong is all... one should run from each lead to ground to create a psuedo-center tap but I'll fix it pronto. Also on main circuit the 2K2 resistor in the tone stack should be a 22K... I tend to like a good bit of treble, tons of bass & a scooped mid. Also I just posted some interior shots if you care to give em' a glance.

[Howlin' Mad Mac]

A smaller V1 coupling cap? What kind of effect will that have on the tone? Brighter? Darker? My big issue is to make the annoying feedback go away... any idea what might accomplish that?

[Howlin' Mad Mac]

Question: Instead of some sort of tone stack I've just seen a schematic of an amp almost the spitting image of this one except it uses an EL84 output tube instead of a 6V6. Anyway the preamp is virtually identical except where the tone stack would be there's simply a 1M resistor across lugs 1 & 3 of the volume pot & a 100K between the coupling cap & lug 1. What's this doing?