KT 88 project request

[nickb]

Hello. Thanks. Those 100 ohm/14 w is as important to keep inrush current under control at startup as well to drop some voltage into primary side for reasons you are talked about.It is a 'soft start switch'. Into a standby mode there are 35v dropped under it so a big power resistor is compulsory for 14w dissipation. I used a 50w aluminium housed (14w without radiator) so it is right on spot for few seconds I waiting before put the switch on.

...well I thought someone will ask me in the end. It is but is useless and for this reason I didn't figure. To explain myself in my country the mains for domestic use... simple haven't. The earth protection are reserved for mains plugs just for kitchen and bathroom. ...idiots.
And Yes fun all the way
Cheers. Catalin

The 100 ohm may drop more than you expect. For example, with a 100W the load the peak current in the primary will be around 2.9A. Also thats 1.1A rms or 45W. Arcol specify a heatsink of 535cm^2 x 1mm aluminium for 50W resistor. Far East ones may differ, significantly. I'm really not sure you need it for surge limiting since the the surge is just a single cycle. If it really bugs you a NTC thermistor or relay might be a better choice. Switching it manually seems kinda clunky and will sooner or later get forgotten leading to damaged tubes.

Interesting about the safety earth. Yikes. The PT screen also acts as an additional safety barrier but only if you have the earth available. Where do you live?

[catalin gramada]

The story is happen in Romania. Regards the limiter resistor is exactly as I said, measured not deducted, for my idling settings condition.
Yes an thermistor will be a better safe option completed with a zener network to limit voltage for preamp whilst filaments heat as R.G. recommend it. It is over my head for the moment how to implement it right on.
Btw. The tubes are still in safe I was assure by some tech. It will not arcing. It exist a risk just for the caps but it is time limited till the preamp tube will heat.usually fast. That means a startup protocol failure. The real problem for preamp caps starts when have not preamp tubes in position. that I can agree. Wonder how many Fenders with underated caps from this point survived over years by owners abuses?

[nickb]

The story is happen in Romania. Regards the limiter resistor is exactly as I said, measured not deducted, for my idling settings condition.
Yes an thermistor will be a better safe option completed with a zener network to limit voltage for preamp whilst filaments heat as R.G. recommend it. It is over my head for the moment how to implement it right on.
Btw. The tubes are still in safe I was assure by some tech. It will not arcing. It exist a risk just for the caps but it is time limited till the preamp tube will heat.usually fast. That means a startup protocol failure. The real problem for preamp caps starts when have not preamp tubes in position. that I can agree. Wonder how many Fenders with underated caps from this point survived over years by owners abuses?

35V measured how? What was current in the heater winding and the HV DC current measured after the first filter cap ( add the two paths in your sch). It's a tricky measurement to make due to the pulse shape of the waveform in the primary. You would need to have a true rms meter to get the right value. The peak will be getting on for twice the rms value. You can find the peak in a couple of ways. Build a little half wave + capacitor rectifier across the resistor, i.e. effectively a peak detector and measure the DC, or easier, you can infer it from how much the DC output on the secondary drops with and without the resistor times the PT turns ratio.

Or there is the fun way: plop your resistor into a 1L cup of water and measure how long it take to warm up by 10C and calculate the dissipation from that.

My concern was if the voltage drop is big enough that the heaters run cold then the cathode coating will gradually be damaged due to lack of migration of barium and the buildup of cathode interface resistance. I don't have a good handle on how low is too low but the data sheets specify no less than 5.7V so that's the figure you know is safe. Just move it to the secondary and it's not an issue. I'm sure it not a big deal unless you leave the resistor in place for a long time and the voltage very low. On reflection the peak drop happens for a fairly short time at the crest of the waveform so it won't reduce the rms heater voltage quite as much. Just pull out your true rms meter and measure it at the heater.

I don't think the common bias/HT ground is a worry assuming you've done the grounds correctly as far as hum is concerned. The common path does mean the bias varies with the HT load due to the drop across the common resistance but that resistance is tiny so the effect is negligible. You are designing around a non-problem here.


BTW, I have learned that some Far East meters labelled true rms are not. Just like the fake CE marks. The funny thing is it's illegal to export "faulty" goods to China. Double standard, huh?

[catalin gramada]

The voltage was measured direct over limiter resistor after 10 minutes in standby with mine Fluke 287. I can trust in it as true rms. Regards phenomena you.re talking I can just give credit to those tech who said it really doesn't matter by cathode poisoning for receiving tubes supplied down than 1kv so I have not care aso.For this reason I gave up to use the regular stand.by switch to allow to heat the heaters first. I used those 100ohm resistor instead to protect a little bit the caps and to can more precise dimension my HT fuses which I think I can give a chance to be more effective in this way. I.m not in position to judge if is right or not so I apply the HT voltage without care if the valves starts to conduct or not.
These are not my conclusions. I used just information I read.including on this Forum.
If is better to move the limiter onto HT side.I.m ready to do it. But as time we are talking in terms of current I don.t think is matter too much in what side it is placed. Thanks. Catalin

[nickb]

The voltage was measured direct over limiter resistor with mine Fluke 287. I can trust in it as true rms. Regards phenomena you.re talking I can just give credit to those tech who said it really doesn't matter by cathode poisoning for receiving tubes supplied down than 1kv so I have not care aso. I.m not in position to judge if is right or not so I apply the HT voltage without care if the valves starts to conduct or not.

Your buddy is thinking about cathode stripping. That is a different effect altogether and not a concern.

[catalin gramada]

So are you saying moving in HT side it is a better option,please?

[nickb]

So are you saying moving in HT side it is a better option,please?

I doubt it's really a problem in practice but it's easy to find out. Measure the rms heater voltage to see. If it's a lot less than 5.7V then you should move it.

[catalin gramada]

Allright. Got ya. Thanks for you nice instructional conversation.
Have a nice evening. Bye.
Catalin

[loudthud]

The power supply in post #41 needs to be simulated. IMHO there is too much capacitance, the result of which will be high RMS secondary current. With a 1.5H choke, ripple on the main B+ will be something less than 1V peak to peak at full power.

[catalin gramada]

Hello. I don't know how to do the simulation. This PS was allready done and it works in my amp. If there are any issues and can help with some sugestions I.m very interested to know and to optimise it. But I cannot provide right currents in this moment. just from my memory...Thanks.

[nickb]

Hello. I don't know how to do the simulation. This PS was allready done and it works in my amp. If there are any issues and can help with some sugestions I.m very interested to know and to optimise it. But I cannot provide right currents in this moment. just from my memory...Thanks.

Well I did and here are some numbers. I had a suspicion from your answers that the exercise was rhetorical.

With a load of 200W

Power amp load voltage: 567V
Power amp load current used:355mA
Power amp ripple: 79mVpp
Preamp voltage (after inductor): 579V
Preamp load current used:19mA
Ripple current in first filter cap: 763mA
Peak current in diodes: 2.45A
RMS current in diodes: 583mA

The impedance of the transformer limits the rms current. If you increase the capacitance e.g changing to 10,000uf increase the diode rms current by just 0.5mA.

The transformer peak to rms ratio is much higher than I said - it comes to 3:1. I was thinking about the Idc to rms ratio. Duh!

[KevinOConnor]

Hi Guys

Hammond states explicitly that the "Classic" series PTs can be used with a bridge over the CT and one end, the other end taped and stowed, and the PT will operate cooler than with the usual half-bridge. They say the only reason they build these with a CTed winding is because customers want it this way even when they are not using a tube rectifier. To me this is the height of lethargy.

For years I nagged Hammond to add bias windings to this line and eventually with the introduction of the 300-series they added a bias tap. This is a gross impairment for the grounding if you want quiet operation. Besides that, it perpetuates the "bias supply as an after-thought" mentality that pervades tube amp construction and design. Worse still is that they decided on ONE value for the tap voltage for the whole line. A 50V tap might produce 70Vdc, which is fine if the end voltage of the winding is 300Vac producing 430Vdc. But... on the 378CX there is 400Vac producing up to 570Vdc. The 70Vdc bias supply is anaemic and won't allow turn-off of every tube sample installed in the amp. For all of these reasons, it is much preferred to add a small auxiliary PT to provide a proper-sized raw bias supply of medium to low-impedance.

Even for "best linearity" the cascaded chokes arranged as the usual proportional supply will provide much reduced ripple to the earlier stages than having the fanned-out arrangement. You can test this by moving one wire.

There is absolutely no problem with high-capacitances in the plate supply. Most guitar and bass amps are under-filtered and i would warn against using fender or others as the icon of good practice.They are definitely the icon of economic practice and clean guitar tones. Hifi practice uses high-value filter caps and not only because some hifi designers are nuts for Farads but because there is benefit to having the higher energy storage for music reproduction.

Have fun

[catalin gramada]

Hello. I did a sketch of my amp. The drawing did not reflect the actual configuration which have the tone control after first voltage amplifier. I want to move it exactly as my drawing shows. First voltage amp it is still a 'researching' subject for me so I didn't figured values. I want to try more tubes types first. just for tone consideration.Paralleled connection for first valve it is another option in case need more gain.
Please let me know what are you think about. Thanks. Catalin


late edit-I figured exactly the points where decoupling caps was applied

[Tom Phillips]

Your sketch does not show decoupling caps (Filter caps) on the pre-amp power supply notes. If, in actuality, they are not there then you should add them.
Cheers,
Tom

[catalin gramada]

Hi. It is another sketch with the power supply in this thread. I did not figured here just to keep the drawing simple. If you search some posts below will find the whole P'S I used. Thanks. Catalin