Alex Kitic

Just as I wrote above, installing an NTC similar to the CL80 or CL90 will not solve because: 1) these are rated for much higher current (so will not limit anything in particular because all the current a tube can draw will pass thru anyway), 2) not being adequate these will not heat enough to significantly change their resistance (becoming a very small resistance that literally disappears in the circuit). The fact that he bypasses the CL80 with a switch confirms 2), and also confirms 1) which is obvious from the datasheet anyway. Instead of the CL80 you could install a 120 ohm resistor (resistance of CL80 or CL90 at 25 degrees C) and a switch to bypass it once the amp is warmed-up. Just like the L80, the 120 ohm resistor will not limit the current relevantly, as the anode voltage will just be slightly lower as per Ohms Law. The NTC would need to be rated for maximum 200mA, constant 100mA and have a considerably higher resistance at 25 degrees C x i.e. at idle. If this problem is caused by cold tubes receiving high voltage too soon (which I believe is not the root cause), a viable solution would be to install a switch that would allow you to power-up the amp without the B+ hitting the tubes, and to switch on the B+ once the tubes have been properly heated (let’s say after 1 minute). The switch would need to be able to withstand the B+ voltage, let’s say good for at least 500V DC. Edit The switch should be placed between the CT of the HV secondary and ground. I believe the schematics calls it red/yellow, but you should check which one is really the CT of the HV secondary. The switch must be well insulated and able to withstand at least 500V DC (better go for higher and keep in mind you need the DC not the AC rating usually found on switches).

While adding an NTC helps with current inrush for whole amplifiers, power supplies, or just heater supplies - the thermistors you mentioned are for a minimum current flow of 500mA which is way above the normal current draw of one side of this PP pair. It would possibly not make any difference (not really slowing startup) but would represent an additional resistance between anode and primary of the OPT with adverse effect (I don’t think it would heat enough to become the .7 ohm resistance mentioned in the datasheet - CL90 is 120 ohm at 25 degrees C and maximum current is 3A - good for 2A3 heaters but not anode current of a tube that will not draw more than 100mA). In addition to that, if there’s something wrong with the connection of g1 in the socket, sooner or later this will strike and the g1 will remain without ground reference, rising in potential and causing the problem. If that’s the cause, which is my first impression. That’s why I suggested measuring the g1 voltage at startup for both tubes in the pair with 2 multimeters (obviously tied to g1 and ground so there is no touching and full attention to the readings).

By the way, guys - have you noticed all my posts mentioning how my RH84 schematics was shown by Matt Formanek as his own (for an 6V6 amp that would not work that way anyhow, at least not for long due to the g2 voltage being too high...) have disappeared? That was my reaction to this post: On 11/16/2015 at 1:44 PM, ToolShedAmps said: That's strange, isn't it? How does it make look - from your view?

Thank you for posting this. I was aware of the thread but did not want to post it myself. Are you still using the RH84 you have built? And, have you considered modifying your console amp (the one painted white) into an RH84? Many people have modded these Magnavox consoles to RH84 with considerable success.

Thank you, Jeff, for putting this in a simpler way. I am always striving to use as little words as I can and sometimes my interlocutors don’t get it right. While this is obviously an intermittent issue that occurs on startup when tubes are cold, by checking all these details we can narrow it to the most likely cause. The main hindrance here, and if I may, the point where this design was either not taught through properly is the bias common to both output tubes - you know if the pair is drawing the correct current but not if this current is correctly distributed between the two tubes.

I guess you need first to measure the DCR of the 2 sections of the primary for each tube, write it down, and than measure the voltage simultaneously with 2 multimeters between B+ and each anode. That would show inbalance as well as give an exact measure of the anode current. What I would also measure would be the g1 voltage of both tubes, with 2 meters, simultaneously.

Whether there is a signal (music) or not during start-up should be totally irrelevant for the proper start-up of the amplifier. This is obviously an erratic issue that does not happen every time and is not repeatable. It’s not a sequence that can be reproduced. The issue must be caused by a component that is malfunctionig occasionally, and is related to g1 on V5 - either the connection socket, or the grid negative voltage (bias) being wrong for some reason. This is made more complicated by the design where bias setting components are common to the pair of tubes, and the cause may be with the other tube in the pair - a lost anode connection means that all the current is drawn, at least for a short moment, by V5. The condition is not necessarily detectable by looking at the bias leds, nor listening to the music signal, and probably happens all the time but not necessarily long enough as to cause the arcing as an evident event. Probably corrects itself once the tubes heat-up and expand. I just cannot shake off the possibility of a bad socket. I have seen socket issues happening with new sockets - never used and new production - with tubes with seemingly perfect pins...

Hello MechEngVic -- any news? Did you find the time to test some more, in particular those bias setting variable resistors?

Hello McEngVic, I took a quick look at the schematics and I understand why the coupling caps should be 630V - at startup the ss diodes are much quicker than the tube heaters, and voltages probably reach 500V or more, and the “upper” caps (like C25) probably suffer the blow. The first thing that comes to mind is that the bias circuit is a voltage divider. If the values are incorrect (like replacing grid bleeder resistor 270k for a different value) the correct voltage cannot be obtained with too high or too low current. Thus make sure that all values are original - 270k should stay 270k unless you make other changes. Also, these 2 resistors need to be the same value because they are part of a shared circuit for the 2 tubes (which is definitely a design flaw worthy of modification). Another question would be about the variable resistors (pots) used to set the bias voltage: I expect those have been replaced? A bad scraper can do awful things in that position, and the issue you are describing looks like a powered tube loosing g1 reference! Last but repeated concern, not only the cathodes are connected to ground through a shared small resistor, but the g1 bias circuit is parallel to both tubes and thus shared. The le-diodes tell you if the voltage is high or low (current low or high) for the pair - not just one tube. What is perfect for the pair is probably twice the admissible current for one tube with the other tube drawing little or no current? Therefore the problem is not isolated to V5 but includes what is going on with the pair tube. The problem must be caused by erratic g1 condition on tube V5 - either lost contact or erratic bias voltage. You can also contact me directly, I’ll try to help. But we should post the solution here for others to find

Thank you for your input. That is exactly what I thought. In my view, this gives an idea of the kind of seller involved (not serious, just grabbing the money and flying to the next deal). Also, most sellers do that, so this falls into the “normal” category, I guess. It’s the ugly side of everyday life.

Hello, I am no Dynaco servicing expert, but I can see that most advice you are getting is rather voluntaristic. There is absolutely no need for 600V rated coupling caps, but they cannot hurt. Also, the size (capacity) will not hurt anything but the sound - and smaller caps tend to sound better if large enough not to cut off the deep bass. The problem you are experiencing is definitely related to the g1 pin of tube V5 - something erratic happens with the bias voltage. Red plating obviously means too much current but arcing is a sign of overvoltage. You should definitely check the biasing circuit and it’s caps (C19 if I am correct). Even if the cap was replaced there’s no guarantee it was good... Also, since the two tubes are basically tied together, one will draw more current if the other doesn’t draw enough... led lights are cool but you should check the bias voltage on the pin during startup, on both tubes. Tubes that have arced are not necessarily good only for the bin and you should reuse those until solving the key problem, possibly using it in the other socket. Last but not least, tube sockets can cause these issues and replacing with new ones is a cure if the new ones are good - which they easily might not be if current production. I guess there’s no need to check the tube pins because you are using current production tubes and not NOS (Nasty Old Stuff that has been rusting for decades in some lonely warehouse)... ? Keep in mind that the other tube in the pair could be causing the problem to V5 by intermittently loosing connection with anode or cathode or... Please recheck along these lines. The pictures and schematics posted are fragments and your attention is being directed at things that either did not exist in the original and working amp. The problem is in something that was there from the start and has felt the bite of time passing by...

These MUST be fakes... never heard of Philips 5U4 and the writing is totally wrong font... plus etched (burnt in) codes are nowhere to be seen, and that’s definitely a sign the only Philips on that picture is the writing on the boxes. The price is exorbitant to say the least - for 5U4Gx (should be GB but these are definitely fishy. Another thing I don’t understand is why the ad mentions several tube types that have nothing in common besides being popular?

Hello Mike, The Cary SLI-80 is an integrated amp with KT88 tubes, and all tubes used in it are also indirectly heated - this means that at startup the B+ will rise very fast to levels higher than operational, and drop some 20-30 seconds later gradually to the operational level. It’s strange that a direct heated rectifier tube (actually 2) are applied here, as the circuit would technically benefit from an indirect heated rectifier like the 5AR4/GZ34. The issue would be that B+ would result at least 15V higher with 5AR4/GZ34 and this might be a problem if anode dissipation is already high for the output tubes. on the other hand, besides 5U4 types, you could use 5R4 types which would result in lower B+ and possibly slightly lower output power - but would impart a different sound signature that you might like even better. The russian 5C3S is similar to the 5U4 and if from a good batch (old SSSR production) might be absolutely interesting given the lowish price. On the contrary, the CAD300SE has got 300B direct heated output tubes, thus the tubes will start conducting almost at the same time a direct heated rectifier would. The CV378 aka GZ37 has a slightly higher voltage drop than the 5AR4/GZ34 but you should be safe with the latter. On the other hand the only issue you might have with 5U4 could be 3A heaters vs 2A for the GZ3x types - which should not be a serious problem because we are talking about 3VA that are certainly not going to tax the power transformer too much. 5U4 should be safe to use here as the B+ will be slightly lower, and the 5R4 is absolutely safe to use (2A heater), but will yield probably 30V less than the GZ34 and you will loose some power - which in your case with the La Scala is definitely not going to be a problem. Bottom line is that you should be able to use any of the mentioned rectifiers in your 300B amp, while the integrated is slightly pickier - although the same rule might apply. What you should do is “roll” rectifiers and base your choice on the sound you are getting. I have little experience with current production rectifier tubes, and would advocate either NOS US tubes or NOS Soviet tubes (including the Sovtek jugged edge anode GZ34). I guess you should avoid the Phillips GZ34 as it is too expensive compared to its sound quality - unless you have money to throw away (we only live once) or are a collector.

I find it amazing that after so many posts a lot of advice was given without asking a very important question - what amps are those? I mean manufacturer, model... Another interesting fact is that the discussion was about the technical characteristics of the rectifiers (some of which are current production, therefore quite approximate in their ratings - if I may). The accent was not given on the sound said rectifiers will impart on the amps. Once I know the application, I can give advise - which of course doesn’t seem be needed in this thread - as the OP has already procured some replacement rectifiers?

And it works for many manufacturers - but it is sub-optimal. If everyone did a perfect job, this would be a perfect world. It depends on the application - hun would be painfully audible with efficient speakers, or if not masked by other issues (stronger ground loops, inefficient PS filtering, hum from direct heated tubes... and a lot of other hum/noise/buzz sources too often neglected not only in DIY but products as well).