5F1 New build

[mstang01]

Hi,

I just finished my first build (a 5F1 clone) and I just don't think it sounds very good... I took the standard 5F1 schematics and layout (and cap values) using Mercury Magnetics iron. I added a 5E1 choke (in the same location as the 5E1). I changed the heater wiring to a DC standoff setup (about 35VDC). The Mercury PT didn't have a center tap for the heater so I used two 100 Ohm resistors from the two heater wires going to my DC standoff. I also added a 470 Ohm screen resistor between pins 4 & 6 on the 6v6 socket, and everything that would normally go to pin 4 (the OT and the choke) are connected to pin 6.

I had also made a star ground going to one of the transformer lugs, but I had a lot of noise. I saw a lot of advice on here to separate the grounds so I separated grounds for the preamp ground and the on/off/volume switch and that really helped with the noise (I still need to get some shielded cable for my input).

The problem I'm having is that the clean sounds fine but it distorts pretty early and the distortion sounds crappy (I guess compressed would be a better way to describe it). I don't really know what a 5F1 should sound like, but I don't think this is it.

I'm using a Sovtek 5Y3 a JJ 6v6S and a JJ 12AX. I had built a bias test cable and with that plugged in the 6v6 socket I'm seeing about 42 mA which I think is high. I also noticed that voltages at my second and third filter caps seem much higher than the listed values on the layout and I wasn't sure if I needed to bring those down. (my diagram showed the values at the three filter caps to be 340V, 295V, and 250V while I'm seeing 358V, 340V, and 290V).

Sorry I was so long, any advice on what I should check would be great.

Mike

[tubeswell]

Hi there

Tweed Champ and Tweed Princeton style amps typically dirty-up from around about '2.5ish' on the vol dial, but you can tweak the circuit, such as for example:

1) beefing up the reservoir cap. What is the capacitance in your circuit for the rectifier reservoir? (You may benefit from beefing it up to around 40uF, which is about the max that a 5Y3 can reasonably take).

2) lowering the pre-amp gain. Try subbing a lower gain tube in the pre-amp - like a 12DW7 or a JJ ECC832 or ECC823. You could also decrease one or more of the plate resistors in the pre-amp to 47k, or increase the cathode resistors to 1k8 or 2k2

3) Changing the frequency response to get some of the bass fluff out of the signal. Try decreasing one or both coupling cap values to 10nF (.01uF), and try reducing one or both cathode bypass caps to 2.2uF or 1uF

3) increasing the clean headroom by other means - such as, decreasing the size of the pre-amp supply resistor at the pre-amp filter node to 10k (or 5k), or decreasing the GFB resistor to 18k or 15k or 12k etc, or lowering the reflected load of the OT (by using a different configuration of speakers plugged into the secondary in such a way as to lower the reflected load to 5k-ish), or subbing in a GZ34 (instead of the 5Y3) to beef up the B+ (you might want to increase the reservoir cap to 60uF if you do this (and you might want to have a JJ6V6S in your amp so it can take the higher plate voltages, and you also might want to tweak the pre-amp supply resistor value to get the amp to dirty/clean-up where you want it to more.

There you go - 2CW of ideas

[MWJB]

358 at the first filter cap isn't particularly high, if you want more headroom try a 5V4 rectifier, maybe even a GZ34/solid state, as Tubeswell suggests (the higher voltage you run the amp at the cleaner it gets). If the 6V6 plare voltage ends up at much more than 410-420vdc (to ground) you may want to try a 660ohm cathode resistor instead of 470ohm.

42mA for the plate current is not particularly high either.

Rather than reducing plate resistor values, I'd think about reducing the values of the 10K & 22K power supply resistors to bring up the B+ at the preamp tube.

I'd actually think about increasing the value of the 6V6 cathode bypass cap, try 100uf/100v.

5751 might also be a useful suggestion for V1.

[loudthud]

What are the voltages on pins 1,3,6 anf 8 on the 12AX7?

[Howlin' Mad Mac]

Missing cathode bypass cap

Recently built a 5F1 circuit myself from all new, quality components & when finished experienced what I think were similar results. Sounded ok turned way down but once it started to distort started to kind of "fart"? The way I fixed mine was to add a 25mfd/50v cathode bypass across the 1st gain stage 1.5k resistor & made a world of difference in my build. Not saying it'll fix yours but it's something that'll only cost about 2 bucks & take 15 minutes to try. If you take a close look at Champ schematics prior to the 5F1 & BF/SF schems they all seem to include this cap... only the 5F1 Champ is missing it but I was told by a pro that schematic is incorrect & it should be there. Any thoughts?

[MWJB]

Many pre BF Fender amps don't match the schems, they didn't draw a new schem everytime there was a revision to a circuit. If they ran out of the designated part, they would often use the closest value/type. Same with most mfrs in 50's & 60's.

[tubeswell]

Recently built a 5F1 circuit myself... Sounded ok turned way down but once it started to distort started to kind of "fart. ... Any thoughts?

Beefing up the reservoir cap to 40uF also helps with this. (Most 5Y3G can take 40uF, even tho their tube datasheets invariably say 32uF or 16uF. If that concerns you, put a 1A 1000V diode - 1N4007 - in series with each side of eth Ht winding and the rectifier anodes - banded end pointing towards the rectifier anodes. This will protect your PT if the rectifier shorts.)

[Howlin' Mad Mac]

Perhaps a little off topic here but a question for you... Getting ready to start another build based upon the Brownface 6G2 Princeton. When buying up parts for my build I decided to take the supply caps off the tagboard & got a pair of JJ 32/32 cap cans. Only needing 3 30mfd caps I've been toying with the idea of adding the 4th cap in front of the others & adding a 5h inductor to the mix. In my vision I would make my first supply tap behind the coil & 2nd, 3rd taps as per Leo's original schematic. Think this would be a good thing or something I really don't want to try?

[Howlin' Mad Mac]

Also while I'm asking away, your suggestion to add some 1N4007 diodes... sounds like a back up ss bridge if I'm reading your installation notes correctly. Cool idea but does this change the voltages or anything?

[tubeswell]

Also while I'm asking away, your suggestion to add some 1N4007 diodes... sounds like a back up ss bridge if I'm reading your installation notes correctly. Cool idea but does this change the voltages or anything?

No it doesn't change the voltages as long as the rectifier tube doesn't short (only then will the voltages become like SS rectified voltages). Each diode half-wave rectifies the signal from the HT and drops the voltage by about 0.6VAC, which is nix in the scheme of things. The rectifier tube doesn't translate any difference in the voltage, and because only the positive half of the VAC cycle gets through the rectifier tube anyway, then the tube output is the same (including the same voltage drop through the tube rectifier) as it would be without the diodes there.

[tubeswell]

better to start a new thread for this and post a schematic with it.

I think you are talking about using a CLC filter in front of the OT power rail??

[MWJB]

"Think this would be a good thing or something I really don't want to try?" I'd stick your inductor after the B+ supply & before the screens, like BF Deluxe, wire 2 of the 32uf in parallel for 64uf at the main B+ stage, add discrete screen grid resistors @ 470ohm 1W.

Otherwise the input choke will make the amp sag, 6G2 already have a 5Y3 (B+ around 370vdc)...later BF models used GZ34 & are much tighter, better headroom (B+ around 405vdc).