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qy5-500 tube

J

Johnnybgood

New Member
Nov 15, 2013
40
1
8
Hi all, I am running a grid driven QY5-500 TUBE with zero bias and the screen voltage is derived from the rf drive into G1 for now.
The age old problem of low output on 15 and 10m has reared its head. The input into G1 is via a 50R matching toroid transformer and I have 3kv on the anode.

Output on 80m is 500w, 40m is 425w and 20m is 400w with 70w drive. Now ofcourse this should be much lower with 5 to 10w drive.
I am not a amp expert, so after fighting the low output on 15/10m for several monthS, I was wondering if grounded grid configuration would improve things. There is no neutralisation as the tube is very stable. I was contemplating a tuned input network instead of the toroid matching, but no sure if this would make a difference. The tube filament voltage is 9.9v@10A and screen voltage is 500v. G2 is bypassed to ground via a 33k resistor and a .01 cap in parallel. The tank coil taps for the higher bands appears to produce a very small dip on a GDO. 80m tap is on a seperate coil which I think may be affecting the higher frequencies.

Other than 15 and 10m, the amp is running very well.
Any ideas how to conquer 15 and 10m and what to look for.

73
John
 

One thing I am checking is the gap between a dividing alu wall and the plate capacitor. However, the stators are already on the dividing wall side, but thinking the rotors are also about 10mm from the dividing wall when on minimum tune position. I have now removed the capacitor and will place some spacers in between the capacitor and the wall and then check again on 15 and 10m. I have also moved the tank coil closer to the plate choke and tube, which gives me shorter band switch lengths. If this fails to make a drastic improvment, then I will have to use a vacuum capacitor with a minimum capacitance of say 4pf and a maximum of 150pf.
 
There could be a few reasons why the amp is behaving like this. You will need to test a few things in order to determine what section of the amp is causing the reduced power. Tuned input circuits are generally more efficient than broadband transformers when tuned correctly. They also reduce IMD, harmonics and drive power requirements. Now lets see if you need one.

Does the amplifier draw similar plate current on 10 meters with the same drive applied as the other bands? If the trouble is the input circuit, less drive will reach the control grid and plate current will be noticeably lower. If the plate current is higher, we can move to the next step.

The tank circuit is probably not matching the resonant load impedance of this tube on the upper bands to the 50 ohm line. Finding a dip on the grid dip meter simply means we have resonance. It does not tell us what impedance the tank has stepped up the 50 ohm line to or if that matched the output plate impedance of the tube.

This can be easy to correct by using the bands that are working to determine the actual plate load impedance this tube is producing in this circuit and match the bands that are not working to the ones that are. All it takes is an MFJ antenna analyzer and a 5000 ohm carbon potentiometer with short leads to minimize inductance.

Load the amp up "full tilt" on 80 meters. Power EVERYTHING DOWN. Remove the tube. Connect the potentiometer between the anode clip to the tank and chassis ground AS SHORT AS POSSIBLE. Connect the MFJ to the amp OUTPUT connector and MANUALLY engage the RF relay with no other power or drive applied. Now adjust the pot for a flat SWR on the MFJ with the MFJ set to the same frequency you just tuned the amp on.

Now switch the amp to 10 meters and the MFJ to 29 Mhz. Adjust the load and tune controls of the amp for the lowest SWR without touching the pot. You're likely to find adjustment of these caps cannot tune a good match because the 10 meter tank coil is the wrong inductance. I take a ferrite bead glued to the end of a drinking straw to determine what way the coil has to be modified.

If bringing the ferrite closer to or inside the 10 meter coil reduces SWR, you need more inductance in your coil. Compressing the turns together may help enough if you're close. If the ferrite makes the match worse, you have to much inductance. Spreading the turns apart may be enough here. Tank coils should also be of thicker material with more surface area on the upper bands like 15 and 10 to reduce heating and loss associated with circulating currents and the skin effect.

You should also check that the plate capacitor is not too far meshed and adding excessive capacitance to the plate. At 10 meters you don't need to add much external capacitance to the tubes internal capacitance and it should not exceed 40 pf. That's almost fully open on most caps that can work 80 too.
 
Thanks for the reply. I removed the plate cap to cleaned it out and found that the two brackets which hold it fast were heat fatigued and snapped off in my hands. So just waiting for some to arrive in the post via ebay. There is plenty of drive as seen on the grid current meter on 10m and good on 15m. Once I get it all up and running, I will get back to you.

Before I took it apart, I did connect a mfj to one ant port and a 50R load to the other, then engaged the relays, with the tube in place. On the lower bands, I got a good match but very little, if anything on 15/10m. Moving the tap positions on the tank coil produced no match at all. On 10m I have a 4mm copper coil 4 turns at 55mm dia and a 5mm silver flat strap to the band switch.

I will get back to you once the brackets arrive.
 
I have now re-assembled the tank components again and rewound the 10m coil with 3.5mm copper wire spaced wire thicjness and 5turns. 15m came in straight away as did the 10m band. Now I just have to add the 80m coil and see how far it will shift the higher bands, if any.

Thanks for your help as it was dead easy your way.
 
80M is also ok as is 50Mhz, which is surprising. Is this the norm for a HF amp to work on 50Mhz when not really designed for that band?
I have a length of 10mm silver plated copper strap from the tube anode to the two dc blocking caps which is about 3 1/2 inches long and the 50Mhz tap point is approx in the centre of this strap.

I have not yet put the chassis back into its casing, so was curious to see if 6m would be resonant.
Another question is that I want the amp as "clean" as possible but with 70w drive, Its over the top for a grid driven amp. Any comments on this ?
 
I would never try and load the 80 meter position for 6 meters. This could never present a proper match between the tube and 50 ohm line at VHF. It's much more likely some other part in the output section is dissipating the power or becoming resonant and will blow right up if you apply power on 6 meters. Could be anything from a parasitic suppressor to a portion of your plate choke becoming resonant on 6 meters.

It will also be difficult to design an amp of this power class to cover 80 through 6 meters with a bandswitch. It sure won't happen by accident as you'll have some serious layout issues to overcome regarding inductance within lead lengths to the bandswitch and plate tuning cap.

Clean as possible only happens when many more parts are added inside the amp than what you need to make power. If that was the goal, you have to do things like install tuned input circuits that attenuate harmonics before allowing them to become amplified, dynamically shunt regulate the screen grid and bias to reduce IMD. Neutralization also reduces IMD in tetrodes even if they run stable without it. Pi-L tank circuits should be used in the output to further reduce harmonic output. These things make the job much harder.
 
Hi,what I meant to say was that I used the 80m bandswitch position just to test and see if I could get 6m to dip, which it did.
Ok, well I put the chassis back into its casing and fired it up.

Still very little output on 15 or 10m. However The lower bands have seen some improvement. The drive is fine but very little dip on the plate cap. Below readings at 3kv Plate voltage into a 50R load. The dips on 15/10m were very good using the MFJ. I will try and get a complete ready made tuned input for the amp as a start and then add a neutralizing cap as well.


Band anode current grid current output
----------------------------------------
80 495mA 70mA 600W
40 420mA 62mA 500W
20 400mA 55mA 500w
 
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Update:

I am making a 10m tuned input and just waiting for some ceramic trimmer caps to arrive from ebay, which I will use in the final circuit. I will use some sm caps I have which are low voltage 50v and see what the MFJ says. Another question on the grid input...Its got a 300R dumping resistor to ground and also a .001 cap to ground which is part of the grid meter shunt. Then there is a 1nf cap to G1 itself.

Will I have to take these values into consideration as well. Perhaps I should simulate the circuit first to see what happens.
 
I moved the plate cap earth strap as close to the tube base as I could ( actually right onto it ) and this has now given me 200w out on 15 and 100w out on 10m. I am looking for some silver plate flat strap with which to rewind the 10m coil. Hopefully that will bring it in even more. No surplus shops in around here, so will prpbably get some antique silver plated platter and cut that up.
 
Last edited:
Here is a question for the experts, What difference if any does the positioning of the dc block anode caps have on the tubes anode impedance. Traditionally, its mounted close to the tube's anode, but what happens to the line if its mounted closer to the plate cap. Is there a difference on 10/15m?

I understand the potential for having HV potential over a larger area and making it more dangerous.
 
First thing is a 300 ohm grid load resistor is very low for a tetrode. That's only a 1 to 6 step up ratio in impedance and will cause the stage to have low gain. That does not explain the loss of power on the higher bands unless the toriod input transformer has lots of loss on the higher bands and would be getting hot. To get good gain from the stage usually means using a grid load resistor around 5000 ohms and gives a drive to output ratio around 100 times. Without neutralization, I'd be hesitant to bring it towards maximum gain and might try bringing it from 300 to 1000 ohms first. I recommend installing a glitch resistor inline with the plate in case it breaks into a bad oscillation. Choose one rugged enough to hold up to a glitch and pop the line fuse before opening or you could over dissipate the screen grid in the absence of plate voltage.

It may not be easy to find a ready made input circuit for a tetrode since most are for triodes working at lower impedances. You'll probably have to make this if you want to install one. Since the grid load resistor establishes the input impedance to the tube, you can use your MFJ in a similar manner as you did to tune the output stage on this input circuit too. With no power applied to the amp and manually engaging the RF relay, you can tune your input circuit to match the grid load resistors impedance on each band.

It's also OK to relocate the DC blocking cap closer to the plate tune variable. Many amplifiers mount the DC blocker right on the back of the tune variable.
 
Hi, Ok I already have a glitch resistor on the HV line. I was wondering if moving the dc block caps would affect the higher bands or not.
I will try a higher dump resistor of 1K and see what results I get.
Thanks again for the info.
 
I fitted a 470R resistor on the input to ground and no sign of any instability in the amp, but a small increase in output on the lower bands. So far so good. Next step is another one in series to ground, which will be close to 1k. This may have to wait until I can get some space of the table. I also replaced the grid current meter with one give me a better range of 0 to 150mA. The previous one was 0 to 50mA and pinging over on tune up and voice peaks...not good.

I was also looking at making a new front panel as I "fell over one evening and bent it a bit on the way down" ..Too many liquid refreshments (y)
 

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