Mission 5F4 schematic or layout

[Rich]

Has anyone a link to a Mission 5F4 schematic or layout?

The Fender, Weber and Ceriatone are different

[Diablo]

Has anyone a link to a Mission 5F4 schematic or layout?

The Fender, Weber and Ceriatone are different

The Fender and Weber look the same to me. The only thing Weber does differently is sub in a 12AX7 in place of 12AY7.

[Rich]

The Fender and Weber look the same to me. The only thing Weber does differently is sub in a 12AX7 in place of 12AY7.

Weber makes provision for adjustable bias which is not the same as Mission or Ceriatone - just one example of why I'd like a peek at a Mission schematic - I have to trace a problem with a second hand build I bought.

[Bruce / Mission Amps]

Weber makes provision for adjustable bias which is not the same as Mission or Ceriatone - just one example of why I'd like a peek at a Mission schematic - I have to trace a problem with a second hand build I bought.

Is this a repair on one of my older 5F4-5E5A-5F7 amp kits?

[Rich]

Is this a repair on one of my older 5F4-5E5A-5F7 amp kits?

Hi Bruce!

The seller claimed the amp was assembled from one of your now discontinued 5F4 kits. "5F4 Tweed Super Replica hand built by yours truly using a Mission kit."

When I first tried the amp it made a motorboating sound which was coming from one of the power tubes. The caused me to take a look inside and mess with the bias for starters (just wanted to see where it was set).

Since looking around for your schematic I think I have tracked down the problem which I originally thought was related to the bias system as I took some readings at the power tube cathodes which didn't look right.

Later on using one 6L6 I got a reading of 0.04V on the brown wire from the OT and then after changing the tubes in their sockets with same tube I got a reading of 0.052V (plate voltage 440v) so the tube was running 17.6W when in V5 and 23W when in V4.

This leads me to believe the OT is faulty.

Across the OT primaries I read 171 ohms, blue to center tap 95 ohms, brown to center tap 76 ohms. So I guess I need a new OT?

I appreciate your interest.

Still would like a schematic of your version if available.

[tubeswell]

Later on using one 6L6 I got a reading of 0.04V on the brown wire from the OT and then after changing the tubes in their sockets with same tube I got a reading of 0.052V (plate voltage 440v) so the tube was running 17.6W when in V5 and 23W when in V4.

This leads me to believe the OT is faulty.

Across the OT primaries I read 171 ohms, blue to center tap 95 ohms, brown to center tap 76 ohms. So I guess I need a new OT?

Some difference in DC resistance is normal on opposite sides of an OT primary due to one side of the winding being wound over the top of the other, and hence being longer.

Have you tested the OT on a variac to measure the reflected load on both sides of the primary (to see if its the same?)?

Did you try another 6L6?

[Rich]

Some difference in DC resistance is normal on opposite sides of an OT primary due to one side of the winding being wound over the top of the other, and hence being longer.

Tried the same test on a new Hammond 20W OT - perfect balance.

Have you tested the OT on a variac to measure the reflected load on both sides of the primary (to see if its the same?)?

Don't have one.

Did you try another 6L6?

When I couldn't get a match with the original set of tubes, I tried 4 other sets 3 of which were new matched sets. The fact I got such different reading with the same tube on each side of the OT leads me to believe I will never be able to achieve reasonable bias with this OT, the question then is there any potential risk in running this amp?

[tubeswell]

Not all OTs are the same w.r.t. DC resistance on either side of the primary. The only way you could test it for sure is with a variac (or some other form of low-voltage AC source attached to the secondary - in order to measure the VAC that appears across each side of the primary). It is the VAC that is critical, not so much the DC resistance

[Bruce / Mission Amps]

"Later on using one 6L6 I got a reading of 0.04V on the brown wire from the OT and then after changing the tubes in their sockets with same tube I got a reading of 0.052V (plate voltage 440v) so the tube was running 17.6W when in V5 and 23W when in V4."


Well I'm not completely sure what you mean by some of those voltage readings unless it is some kind of voltage drop across the center tap to the plate wire while under load... ??


Is it using one of my Heyboer, 35 watt, open frame, paper bobbin wound OTs with 4 and 8 ohm taps?
It should have a white sticky label with a part number on it.
It could have a metallic, red white and blue "made in the USA" Mission Amps label too.
Although the actual batch run might be different, I still use those in other amps and have them in stock so I can measure the DCR on one for you.
In this amp, with the three Heyboer transformers, a NOS 5U4GB rectifier and a good set of 6L6GCs, ...each of the power tubes are supposed to be biased with about 45ma-55ma of idle current.
A matched pair of power tubes should be within 2ma to 5ma in any case.
I wouldn't be 100% sure the OT is bad though... I've used over a 125 of them and this would be the first blown one I've heard about.
Naturally that doesn't mean it is not bad!!
Is it possible one of the sockets is bad, or the 220K grid load resistor is bad, or the actual bias voltage to one socket is off.... etc.
In your Hammond OT vs tube test, did you also clip lead the Hammond OT into this amp? I would try that too... the stock mid 50's 6L6 OT is only a 25 watt OT and most Hammond OTs are under rated so it should not hurt it at all.

[Rich]

One of the reasons I was tying to get a look at the Mission schematics was because when I asked the seller for the schematics and any instructions that may have come with the build, he stated " There was no "build instructions" with the amplifier, just a schematic/layout which I no longer have a copy of." and referred me to Weber and Schematic heaven; drawings from these sites don't exactly match this amp.

I became suspicious, 'Mission kit' (because of your stellar reputation) weighed heavily on me buying this amp.

There are no identifying marks or paper except for the choke which has a white sticker marked HT-6172 9960342 (which I was not able to find on the net), I've never had anything with Heyboer iron but I've never seen these before. The OT does have 4 ohm and 8 ohm taps.

Options are to use my Hammond Iron to change the amp to a 5E4 or get another OT. Consensus on the present OT will force a decision.



I must have mislead you here the way I explained it, I did ignore the small differences in high voltage for ease of explanation.

When I got it, (tube 1) V5 was 440V on pin 3, 443V on pin 4, -43V on pin 5 and 0.040V on pin 8 or 17.6W through tube one;
while (tube 2) V4 was 442V on pin 3, 446V on pin 4, -43V on pin 5 and 0.055V on pin 8 or 24.2W through tube two.

Changing the power tubes around, tube 2 in V5 read .043V on Pin 8 or 18.9W through tube two and tube one while V4 read .052V on Pin 8 for 23W.


So when on the OT brown wire, tube one is putting out 17.6W, but when on the OT blue wire it's putting out 23W.
When tube two is on the OT brown wire it is putting out 18.9W and when on the blue 24.2W.


Because the readings from both tubes were higher when in V4 I concluded the OT was likely faulty but I continued testing by changing bias settings, went to -48V as at higher settings the amp made the noise I hear just before a tube red plates. At this bias setting, plate voltage was 460 and the cathode readings were .031 and .038.

I dug out some spare tubes (in order to totally rule out the tubes) and grabbed more meters - one on the bias, one for each 6L6 plate and one each cathode.

I set the bias for -45V and monitored 4 more sets of tubes (3 were matched sets). I tried out each pair, each tube in both sockets as a pair.

The meters were steady as was the trend for higher output on V4.

My conclusion is that I will not be able to bias any set of power tubes at the nominal 70% setting. When it was suggested I test the primary resistances, the results reinforced my conclusion.

While the reason I came to this site was to try to seek a copy of Mission's schematic, now that I have your attention and you have a fuller description of what's going on with this amp, if you have any thoughts on this matter before I start swapping iron I would be most appreciative.

[Bruce / Mission Amps]

Hmmm... something funny there.
Disconnect the negative feedback wire from the output jack and try again... do the same measurements and record it. See if that changes the results you are getting. It remotely reminds me of a single power tube breaking into an ultra sonic oscillation.
If not, next I'd power down and remove the brown and blue wires from their sockets and flip them around from socket to socket without changing anything else. Fire it up and do the measurements as before.
If it still is doing the same thing with no NFB and the OT wires flipped around to the new tube socket then I would swap out the OT... but I thought you already did that with the Hammond.

[Rich]

Finally got around to changing the OT in this amp.

Bought a Hammond 1750J which gives good readings on the bench, installed it with new 5881 tubes, balances very close just shy of 17W a side, sounds great.

Still looking for the drawing. Has anyone a link to a Mission 5F4 schematic or layout?

The Fender, Weber and Ceriatone are different.

I guess I'd better post this in the schematic forum.

Thanks to all who tried to help out!