Help with a Soldano

[C Moore]

Working on a Soldano Hot Rod 50. The only schematic(s) I can find are the collection below. The V4 cathode runs at 225 volts. Across a 100k resistor that is a half watt. I have a 1 watt metal film in there, but it still gets pretty warm. It is PCB, so I do not think it is possible to get a bigger resistor in there without it looking pretty "funny".
Does this voltage seem normal to you guys.? I do not like how hot this cat resistor gets.
Thank You
http://www.el34world.com/charts/Schemat ... hotrod.pdf

[tubeswell]

should stand up okay. You could always try 150k or 180k and see where that gets your cathode voltage I guess. (might be cooler)

[KT66]

A 225 voltage drop across a 100K resistor means that the tube is drawing 2.25mA of current ( Ohms law - I=E/R...225/100K = .00225) . If you calculate the power ( Watts = voltage X current) at 225 X .00225 = .50625, you can see that it is just a tad over 1/2 a watt, which would explain why the resistor is getting hot.

Sometimes as amp nuts we tend to focus too much on what the guts of an amp look like, I think it would be better to have the right part for the job even if it looks funny, as long as it is electrically solid, so I think you should upgrade the resistor here to 1W. You can find modern 1W resistors that are in a small package that would probably fit fine, like these for example : http://www.mouser.com/ProductDetail/Xic ... 9ysDxLI%3d

[C Moore]

Thanks for the replys.....
But, I have a 1 watt resistor in there now. It still gets pretty warm pretty fast. I do not know how hot it might get after 20-30 minutes of play. I was thinking more along the lines of a 2-3 watt resistor. I cannot recall any other amp I have worked on that had a resistor get this hot, and the amp was working "normally".
Also, as I mentioned in another post, the OT is split real funny. It measures 100 ohms and 78 ohms from CT to each primary. This is another condition I have never seen in another amp. Makes it impossible to get any kind of a "matched pair" of tubes in there.
Maybe I need to shoot an email to Mike Soldano and ask him what he thinks.?
Thanks

[tubeswell]

Also, as I mentioned in another post, the OT is split real funny. It measures 100 ohms and 78 ohms from CT to each primary.

It not unusual for there to be DC resistance difference in OT primary windings where one side's turns are wound over the top of the other. What's important is that you get the same VAC on each side of the primary when you put a (smallish) VAC on the secondary.

[C Moore]

Also, as I mentioned in another post, the OT is split real funny. It measures 100 ohms and 78 ohms from CT to each primary.

It not unusual for there to be DC resistance difference in OT primary windings where one side's turns are wound over the top of the other. What's important is that you get the same VAC on each side of the primary when you put a (smallish) VAC on the secondary.

OK.
I have the time to check. If it is not too hard to access, I will give it a shot. I only have access to 12 VAC from a Wall-Wart. Would that be OK for this test.?
Thanks

[martin manning]

HH, I replied to your other thread suggesting you do this test to see if the difference you are seeing in resistance is not due to some damage to the OT. An AC wall wart should be fine. You can apply the signal at each side of the primary, measure the voltage there and at the speaker jack, and calculate a ratio.

You can do it the other way, applying the signal at the speaker jack, but be aware that the secondary impedance is low and the current draw will be high. A small series resistance might help with that. Maybe try 100 ohms, and a 2W rating would be safe. That would draw 333mA if it were the only load, so see if the wall wart is good for that. The advantage of doing it with the signal fed into the secondary is that you can measure all three voltages with the same hook-up at the same time.

[C Moore]

Martin......
So, I need to lift all the leads first, including the CT and the 0 lead on the secondary.?
Then I apply my wall wart to the Blue and Brown wires of the primary. Clamp one meter lead onto the 0, and measure the 4,8,16 one at a time with the other lead.?
Is that kind of the procedure.?
Thanks

[tubeswell]

Martin......
So, I need to lift all the leads first, including the CT and the 0 lead on the secondary.?
Then I apply my wall wart to the Blue and Brown wires of the primary. Clamp one meter lead onto the 0, and measure the 4,8,16 one at a time with the other lead.?
Is that kind of the procedure.?
Thanks

You should unplug the output tubes and disconnect the secondary leads from the speaker. Measure the VAC between each primary side and the primary CT. A 12 VAC source will be fine for the secondary (but it has to be AC). You will get quite a high VAC on the primary so be careful.

[martin manning]

I think you only need to lift the CT, because the plate leads are open circuit with cold or removed power tubes. If the amp has NFB then you want to disconnect either that or the secondary's ground if you feed the signal in at the primary. If you feed it in at the secondary I don't think it would really matter much since the secondary is such a low resistance, and you can measure the voltage across it directly and at the same time as you measure the primaries. You'd want to use the same tap as the NFB is on, though, and I'd put that 100 ohm resistor in series to protect the wall wart.

[C Moore]

Thanks Again for all the info/advice.
I heard back from Soldano. Bill told me their prototype OT's were the same. Said he had two in the shop that measured 98/77 and 96/76. Merc Mag said they were intentionally designed that way, so as not to be "Hi-Fi" sounding. OK...so the OT is working normally.
I am OK with my 1 watt resistor. Soldano says they use a 0.70 watt in that position, so my 1 watt is OK. It still gets hotter than all other resistors in the amp.......whatever.
But this amp is still "noisy". It is pretty quiet with Pre and Master at Noon. Past that point and this amp gets pretty noisy. I only have experience with a few "high gain" amps, but that kind of seems to be the norm, and is one reason I do not like them. The amp was built in 2003. I do not think the filter caps are dead yet. My experience with bad filters is that they hum as soon as you get any volume at all, and get much worse as the volume goes up.
What do you guys think of an amp like a Soldano Hot Rod 50.? Are noise and these amps just a fact of life.?
Thank You

[martin manning]

Hmm... So they are telling you that the primaries are deliberately mis-matched. That'd produce some second-order harmonics. It'd be interesting to know by how much.

[DonMoose]

I am OK with my 1 watt resistor. Soldano says they use a 0.70 watt in that position, so my 1 watt is OK. It still gets hotter than all other resistors in the amp.......whatever.

I've had 3 different amps - a Boogie, a Fender and a Soldano-designed Yamaha all have a plate resistor go 'foom' this Summer. Based on that sample size, I'd say go ahead and put in something a little beefier as a reliability move.

... or, if it suddenly loses sound or takes on a ratty/spitty character, you know where to start looking.

Hope this helps!

[C Moore]

Hmm... So they are telling you that the primaries are deliberately mis-matched. That'd produce some second-order harmonics. It'd be interesting to know by how much.

Here is the email from Merc Mag.......


Hi Chip,

Thank you for choosing Mercury!

Unfortunately I do not have one to measure as Soldano is not using us
anymore, and we do not spec or use DCR to measure impedance.

I will get with our head engineer next week and have him take a look at the
spec.

It is common and intentional that we do that to achieve tone.opposite of
Hi-Fi. That said, I need to see if this one is that off.

Aside from that, we are in the worst state ever for tubes right now, and to
have 3 sets way off does not surprise me at all unfortunately. 45% reject
rate has sadly become the norm.

Also, as cool as the Bias Rite is, it will never show you what a scope will
for biasing.

I will let you know Monday or Tuesday on the tranny.

Worst case, we would have you send it here for test which is no charge other
than shipping each way.

Thanks.

[Super_Reverb]

Working on a Soldano Hot Rod 50. The only schematic(s) I can find are the collection below. The V4 cathode runs at 225 volts. Across a 100k resistor that is a half watt. I have a 1 watt metal film in there, but it still gets pretty warm. It is PCB, so I do not think it is possible to get a bigger resistor in there without it looking pretty "funny".
Does this voltage seem normal to you guys.? I do not like how hot this cat resistor gets.
Thank You
http://www.el34world.com/charts/Schemat ... hotrod.pdf

Could piggy back a couple of 220Ks or 180Ks in parallel if this worries you. As it stands, you have a about 100% design margin from power perspective. You are ahead of the game relative to commercial amp design techniques.

Relax, plug in, and crank up the volume. Get on the wah if you got one.

cheers,

rob