Primary OT spec for 5F2A build

[Bangy]

I am starting my third 5F2A build Planning substuting a classictone OT, in place of the Heyboer Ive used on the first 2.  Ive discovered the primary impedance of the CT OT is 17,000 OHMs while the Heyboer OT Primary is 7K ohms.

Can someone explain how the primary impedance of the OT affects the circuit?

Im wondering what Leo used? I know he liked to underspec the OT to save the speakers by limiting the lowend that reached them.

Also what would you guys expect to hear as differences?

Thanks for your insight

Bangy

[sluckey]

I am starting my third 5F2A build and I hash planned on using a classictone OT, instead of the Heyboer Ive used on the first 2. Reasons are unimportant, however Ive discovered the primary impedance of the CT OT is 17,000OHMs while the Heyboer OT Primary is 7K ohms.

Where did you find a ClassicTone transformer for a 5F2A that was rated 17000Ω? What model number? The only suitable ClassicTone transformers I see are 8K/5K.

[BetterOffShred]

Like Sluckey says
http://www.classictone.net/40-18031.html

15W, 5k & 8k primary
EDIT:
http://www.classictone.net/40-18045.html
 Listed as 8.5K CT,   BF/SF Princeton, and reverb,  Weber specs this one in their 5F2A schematic.   (022913)

-Brett

[Bangy]

http://www.classictone.net/40-18110.pdf

Here it is.

Its a 5F1 OT, sorry i thought I mentioned that. Oops

Really interested in understanding how the primary impedance of the OT affects the circuit.

Bangy

[BetterOffShred]

That's the "Tiny sized Tweed Champ" OT from Classictone. 

If you want a 5F2A you should look at that second transformer there the 18045.   That's what's spec'd by Webers kit etc.

-Brett

[Bangy]

I spoke to a life long tube amp tech and he said go for it as he has worked on my stuff for decades and knows my tonal preferences.

I am wondering what the impedence 'mismatch' will do, specifically clipping within the circuit and frequency response of the build?

Will using this OT, limit lows, and cause the OT to clip sooner than my builds with the 'proper' OT?

Thanks guys

Bangy

[sluckey]

If you want a 5F2A you should look at that second transformer there the 18045.   That's what's spec'd by Webers kit etc.

That's wrong. Weber does not recommend the ClassicTone # 40-18045 for a 5F2A. That OT is for push/pull amps.

[BetterOffShred]

Oh, well the attached pic is what I was looking at, and the 022913 is what crosses to that OT from classictone. 
https://www.tedweber.com/media/import/w022913sch.JPG    appears to be 8k. 
Not sure what to make of that.

[HotBluePlates]

That's the "Tiny sized Tweed Champ" OT from Classictone.

I was just about to say the same...

The 40-18030 seems to be ClassicTone's "normal" Champ OT (8kΩ primary impedance).

The smaller core of the 40-18110 appears to seek to mimic the same distortion character of an original Champ OT they analyzed.  I'm not sure if the 17kΩ primary impedance was taken from the original sample, or ClassicTone's own change due to what they perceive to be common build issues. I'd expect the higher primary Z to reduce clean output power, though the smaller core will also tend towards saturating earlier than the 40-18030's core.

The 15w 40-18031 is, IMO, a step in the wrong direction unless you're trying to use the 5kΩ primary tap with a 6L6 in a Champ circuit for more output power.  The bigger core (compared to the 40-18030) means that with a 6V6 it will sound "more Hi-Fi than vintage" and may be a poor choice if you want a vintage sound in a Champ recreation using a 6V6.

The primary impedance, along with the available supply voltage, determines the power output (and has some effect on the character of the distortion).

In SE, you might idle at 100% dissipation and assume tube current will swing to a peak of double-idle current.  It must do this while leaving some plate-to-cathode voltage.

Ex.
 -  14w 6V6 with 385v plate-to-cathode (after whatever is dropped in cathode resistor) = 14w / 385 = ~36mA idle.
 -  We might need to leave 60v at the plate at maximum swing.
 -  So the plate current increase of (another) 36mA will drop 385v - 60v = 325v across the OT primary Z.
 -  Ohm's Law: 325v / 0.036A = ~9kΩ primary Z to make this happen.
 -  "8kΩ" is close enough and commonly available.
 -  Power Output = (325v * 0.036A) / 2 = ~6w RMS (which is probably very optimistic; some other limit will probably be hit first)

Your supply voltage could be higher than typical (not uncommon with today's wall voltage).  Your high screen voltage might allow the plate to be pulled a bit lower for a wider voltage swing for the same current swing.  Per Ohm's Law, that implies R (in this case, Z) will go up.

Let's see what happens with 420v plate-to-cathode (after bias voltage)

 -  14w / 420v = ~33mA idle
 -  Plate can swing down to 50v; 425v - 50v = 375v peak swing
 -  375v / 0.033A = ~11.4kΩ
 -  (375v * 0.033A) / 2 = ~6w

Again, 6w is too optimistic for a SE 6V6.  And the tube will not be able to cleanly swing up to 2x idle current and also down to zero idle current.  A higher-impedance OT will have less current swing for the same voltage swing.  Shooting for 12-14kΩ would probably do the trick.

(375v² / 14kΩ) / 2 = ~5w RMS, which sounds more-correct.

Pushing OT primary impedance even higher will drop output power closer to 4w while also skewing distortion to include more odd and even harmonic distortion.  Smaller core size ensures saturation at low output power.

I may have just talked myself into thinking this is a good option...

[Bangy]

Wow! Thanks HBP for taking the time to lay that out for me.  I am going to print your message and refer back to it.  Great lesson and very much appreciated.

I am beginning to wonder if there is a misprint on the classictone schematic. 17K seems awfully high even to this newbie, but (as you say) impelling as well.

The Power transformer Doug uses (And I am using) gives me a B+ of about 320V, It is the Heyboer.  It saves me from using my brown box when playing these builds. I imagine this is his intention in specing 290V for the PT primary high voltage. I believe Fender used 325V.

Nevertheless my output sounds amazing (first two builds). I stuck to Dougs build with few exceptions. Plan is to use the first two in Stereo with effects, and the third (now building) will drive a P12R, and be used gTR-cord-Amp for a lead tone. Its all a big experiment. HAving a ton of fun.

This board is such an exceptional resource -- I hope some day I can actually offer help.

Thank you all again for chiming in, BAngy

[Bangy]

I already heard back from Classictone.

'The ratings for the 40-18110 are correct since they were from the vintage output transformer that we reverse engineered from an actual vintage 5F1 1958 Fender Champ. '

HBP you nailed it!

[Bangy]

I finished the build and this one actually worked on initial power up, now thats a good feeling. I used the classictone 17K primary OT and it is hands down the best of the three I built. The articulation is better, it has wonderful dynamics and allows for your play to determine a great deal of the tone at any volume. Ill do more side by side comparisons but it did seem to break-up a wee bit sooner as we expected.

Ill be finishing the cabs the next couple days and ill be posting pics when they are complete.  Thanks again for all your help along the way.

Bangy

[PRR]

320V B+ with 17K loading suggests 19mA idle, 2.4 Watts output.

Plate near 320V screen near 250V suggests 21V of G1 bias. 21V/19mA leads to a 1.1K cathode resistor (use 1K 1W).

Did you go this way? Or did you go with the usual ~~500r cathode R for about 35mA idle current?

I'm wondering if you run 35mA when you "only need" 19mA enhances tone and articulation.
______________________________

BTW, the 40-18110 part may be a good match to a 6AU6 at 150V for a third watt. Alternate to the Reverb Transformer. Not sure the added $3 is any benefit.

[Bangy]

Hi PRR

I used exactly what Dougs schematic called for 1.5K, however I did bypass the V1B cathode as well (on a footswitch) making the ground for the 25/25 cap. The tone is better all around with the 2nd cathode bypassed or not.

I have another question for you. . . The first build I switched the Primary OT leads so in AMP1 red primary from OT goes to Pin 3 (Plate of the 6V6) and the blue goes to pin 8 of the 5Y3 (hi voltage), on the other 2 builds I reversed this 'correcting' the mistake according to Doug's diagram.  They all sound good, however I will be playing these together and wondering if I have a phase issue waiting to present itself once I finish the cabs and start playing my trilogy of 5F2A's?

[sluckey]

Switching the OT primary leads in that amp is all about insuring the NFB phase is correct within that amp. When using a different OT in another amp you may have swap the OT primary leads to insure proper NFB phase within that amp. Once the NFB phase is correct for any particular amp you are done swapping OT leads.

However, if you will be playing a guitar through TWO amps at the same time, speaker phase becomes important, in the same way that having two speakers in a single cab must be phased together, or you will have a lot of sound cancellation. The easy way to deal with this phase issue is to simply swap the speaker wires on one amp. Even better, include a DPDT switch in each speaker cab to easily switch phase so that all amp's speakers are pumping in/out in phase.

[Bangy]

Great idea. . . Thanks Sluckey. I will be playing all 3 of my 5F2A's at the same time.

I have had speaker phase cancellation before when two different bassmans simultaneously, and it was corrected the next day in the shop with a 9 volt battery.

So if I understand correctly, the OT leads ONLY affect the NFB phase within that amp, and speaker phase is totally separate issue, which I am up to speed on.

[gtrplayr1976]

Glad to see this here. I am going to convert my Pro Jr Chassis to a 5f2a in the near future. Guess I'll be needing both xfmrs.

[HotBluePlates]

320V B+ with 17K loading suggests 19mA idle, 2.4 Watts output.

Plate near 320V screen near 250V suggests 21V of G1 bias. 21V/19mA leads to a 1.1K cathode resistor (use 1K 1W).

Did you go this way? Or did you go with the usual ~~500r cathode R for about 35mA idle current?

I'm wondering if you run 35mA when you "only need" 19mA enhances tone and articulation. ...

I used exactly what Dougs schematic called for 1.5K, however I did bypass the V1B cathode as well (on a footswitch) making the ground for the 25/25 cap. ...

PRR is talking about 1.1kΩ in place of the stock 470Ω on the output tube, not a preamp tube.

[Bangy]

Oh, I used 470 ohm as per Dougs schematic.

Thanks for the clarification. Im new to this party. . .

BAngy

[HotBluePlates]

No problem. PRR was asking if the amp sounded better with the 1.1kΩ cathode resistor for the output tube, or the original 470Ω.

[Bangy]

I used the schematic value on all three builds.
Amp1 Heyboer PT and OT, Yellow sozo's coupling and tone, El Menco 470P Silver Mica bright cap- I used the normal production Silver Mica at first, and really didn't like it at all, once I change to my NOS supply, the high came to life and the ice pick died.
Amp2 Heyboer PT and OT, blue soz couplng, yellow tone, El Menco bright cap
Amp 3 Heyboer PT, Classictone OT, same cap line up as 1 and 2, I mode V1B cathode bypass to a footswitch.

All 1/2 watt resistors carbon comp NOS, larger values were new blue resistors.

Amp 3 is head and shoulders the best.

The plan is to use Amp 1 and 2 in Stereo with effects, and 3 as a lead tone direct gtr to Amp. The cabs are being completed now, tonight I am putting on my last coat of finsh and may have photo's by weekend.

Bangy